Hmm... I started off with Ike in this game because Marth wasn't selectable from the start. I ended up liking him A LOT. Later on, I decided to try Mario and was baffled when I tried to use his Down + B.
After looking for information on this move (and drawing a comparison between it and Water Gun) then trying it in battle, I've finally come to LOVE it.
Comparison:
1) Water Gun charges a tad bit faster.
2) Despite how it may seem, both moves shoot about the same amount of water.
3) Water Gun shoots faster and each single shot sends opponents flying farther, but it starts later than F.L.U.D.D.
4) F.L.U.D.D. shoots slower and each single shot doesn't send opponents too far, but it starts earlier than Water Gun
5) Both moves can be directed up or down, even in the air.
6) Water Gun can be tilted up or down more easily. (Not confirmed)
7) Water Gun can be fired before it charges up and it'll turn into a close-range attack. (Does a little over 10% to Mario)
8) Water Gun has more spread, while F.L.U.D.D. tends to stay in a straighter line.
9) Both moves seem to send the opponent equally far if you hit with every drop of water. I'm not sure how this actually works out, since single water squirts from Water Gun send opponents farther than single water squirts from F.L.U.D.D.
10) Neither move does any damage when charged.
11) F.L.U.D.D.'s push back can actually be used quicker than Water Gun because using it before it charges simply results in a smaller spray of water.
As far as I can tell, Water Gun is statistically better than F.L.U.D.D.. It charges faster, sends opponents flying farther, etc... (F.L.U.D.D. shoots earlier, though) Also, Water Gun doubles as a short-range damage attack (use it before it charges). HOWEVER, F.L.U.D.D. is more useful in the end for one reason... The cape. I want to thank the person who suggested a F.L.U.D.D. -> Cape combo. I was doing it the other way around and it wasn't all too useful for edgeguarding. However, it might be useful against novice players because it turns them around. Make the person face the edge, then spray that person off with F.L.U.D.D.. =P
I suggest that you have a fully charged F.L.U.D.D. whenever possible because you WILL get easy kills with it. (Especially against characters that have poor vertical recovery). If you don't have time to charge it, that's fine. You can start charging when your opponent flies off the stage and even finish charging it before they get back (most of the time)!
The two most useful ways to use this move so far are:
Note: A lot of this is recapping what has previously been said. It's geared toward novices, with some tips that MIGHT help better players.
1) Counter-Push
Water Gun and F.L.U.D.D. both send things flying backward, correct? This means that you can counter anything that's thrown at you as long as its velocity isn't high enough to overcome the push back. This means that you can't counter moves like Shiek's needles or Fox's/Falco's/Wolf's Side + B practically. They're just too fast. However, you can counter many, many projectiles and many, many moves. You can push a person using Skull Bash in the opposite direction to keep him/her at bay, but you might actually be better off pushing in the SAME direction to send him flying way off the screen. (If you can manage it) Slow moves like Ike's B attack and Ganondorf's B attack, etc... Can all be taken advantage of. You can push players that use these off the screen with F.L.U.D.D. and before they have a chance to finish. Also, F.L.U.D.D. > Snake's Grenades, while Cape > Snake's Rockets. You can't interchange the two in this case, because F.L.U.D.D. isn't fast enough to push the rocket while the cape doesn't send the grenade flying far enough (fortunately, you have both!). If you ever come across a projectile that doesn't go far enough with the cape, use F.L.U.D.D. to counter it. (You can charge and release it very quickly anyway) Basically... Counter fast moves with your cape, but counter slow moves with F.L.U.D.D. Squirtle doesn't have this versatility, but he can somewhat (not completely) overcome it because his Water Gun charges faster and shoots more violently.
2) Finisher (Edge-Guarding)
You know those times when you and your opponent are already off the screen and you're struggling to actually hit the person? Well... F.L.U.D.D. and Water Gun are the perfect way to end this. They send opponents going FAR back and they're extremely easy to aim. So... if your opponent is near the boundary of a stage, try using F.L.U.D.D. or Water Gun (depending on your character) instead of a smash attack. If both moves are fully charged, Water Gun does better because each spray of water sends opponents farther. If both are uncharged, F.L.U.D.D. might do better because you can use the push back sooner. (An uncharged Water Gun is a close-range damage attack, if you recall)
So... Suppose opponent is off the ledge.
Edge guarding is where F.L.U.D.D. + Cape really shines. Once you get an opponent off the stage, you want that person to stay off the stage. If that person is no longer able to jump but you hit him/her with an attack (besides the cape, Water Gun, and F.L.U.D.D.), that person will regain his/her Up + B move. In many cases, you will end up helping the person if you don't hit him/her far enough with your attack. Thus, you're better off using a move that doesn't give the person his/her third jump again. This is why the Cape is so handy. This is why F.L.U.D.D. and Water Gun are also handy. F.L.U.D.D. is even handier in unison with the cape! Unfortunately, directional influence can neutralize the two moves, which is why you want a fast move like your cape for following up.
If your opponent is just above the edge of the stage (height-wise), try to wait for the player to waste some jumps before doing anything (UNLESS the player is already far away). Otherwise, your cards are already played and the opponent already knows which way to jump to get back. If you shoot this early, you're most likely going to have to use your cape to finish the person off (which is GREAT... if you're not Squirtle.). Preferably, you should start using these low/no damage push back moves after the opponent wastes his/her third jump. At this point, you can nudge him with any one of the three moves and get an easy kill.
HOWEVER, things aren't always this easy because players (like my little brother -,-) like to stay high and float more towards the center of the stage whenever possible. You have to JUMP to get these people. If they're off the ledge but high up, you have almost no chance of getting a kill with F.L.U.D.D. or Water Gun alone because directional influence is enough to bring the person back. Basically... Players can cancel the push back with directional influence, but they can't go forward through it! The only reason you can't kill someone who's high up using Water Gun and F.L.U.D.D. alone is because these moves stop spraying water after a short while. Once you stop shooting water, the player can begin moving towards the stage again.
So... Enough of this F.L.U.D.D./Water Gun only stuff. Both moves are useful on there own, but now I want to start getting into why I highly prefer Mario for getting these kinds of kills. The cape, as I've mentioned, is what makes F.L.U.D.D. more useful than Water Gun. Too many people think that it's an either or situation. You either use your cape or you use F.L.U.D.D.. This is why people think that F.L.U.D.D. is terrible. Think about it for a second... The two moves both have a similar push back function, but they're completely different in every other way. Off the top of my head, here are some basic differences:
1) The cape has a shorter range.
2) The cape is faster.
3) F.L.U.D.D. requires some charging, but also functions better when charged.
4) F.L.U.D.D. can be directed up and down, while the cape only goes to the side.
5) The cape turns opponents around [important].
6) F.L.U.D.D. propels you backwards when used. (You can use this to knock someone out and recover at the same time)
Why would you stick with one of these? The F.L.U.D.D. -> Cape idea is definitely a step in the right direction, but it can get even better. Earlier, I mentioned that Cape -> F.L.U.D.D. is not as useful for edge guarding. I stand by that statement, but the combo IS useful for something else! You know those people who like to use their Up + B when they're already over the stage? You can use your cape a few times, then finish off with a F.L.U.D.D. burst. Viola! You just KO'ed a person who was trying to be "extra safe". (Well... Maybe. It depends on the stage and the player's location)
There aren't very many "hard rules" for you to follow. You have to adapt to the situation and figure out how to knock the person off the stage before he/she can touch the ground or grab a ledge. In many cases, it's all a matter of using your cape and F.L.U.D.D. in the right order. If your opponent is far from the stage, you should probably start with F.L.U.D.D. (longer range) then follow up with your cape (quick, pushes back, but requires you to be close) or another burst from F.L.U.D.D. (slower, only neutralizes [can't overcome counter directional influence], but can be used safely from a distance) depending on the situation. If the player wasted his/her third jump over the stage, you might want to use cape a few times, then finish off with a burst from F.L.U.D.D. to counter the person's directional influence.
I like these moves because every situation requires you to use them in a different order. It's fun trying to figure out how to get a kill in a certain situation. If Kirby uses his Stone attack, you obviously want to use F.L.U.D.D. for the easy kill. F.L.U.D.D. is a better counter against Fox's Up + B (while he's using it, that is), but the cape is a better counter for his Side + B. Thus, you might knock a Fox player off the stage. If he uses Side + B, you counter with your cape. If he uses Up + B, you counter with F.L.U.D.D.. The beauty of it is that you don't have to stop there. Both moves are fast enough such that you go from one to the other or use the same move in somewhat rapid succession. Oh, and use your Smash moves BEFORE there moves if you're going for a push back kill rather than a knock-out. If you use smash moves, the player gets his/her jump back. If you want a knock-out, F.L.U.D.D. -> Smash Move is really handy.
If you want to see what I mean... Start a training match in Final Destination. (Player = Mario, Computer = Link) Move Link closer to the edge. Move Mario closer to Link. Have Link double jump then use Up + B. You SHOULD be able to KO link with Cape -> F.L.U.D.D. (you have to jump before using cape) even though Link actually started above the stage. Even then, there are several ways you can do this. Here are two of them. Note: You might have to set the speed to 1/2 if you're doing this on your own.
1) As Link is going up with his Up + B, jump twice, use your cape, then use your fully charged F.L.U.D.D.. The trick with this method is to aim straight down, then slowly tilt your controller up. This will allow more water to hit Link, ensuring a kill.
2) As Link is going up with his Up + B, jump twice, then fast fall down and quickly use your fully charged F.L.U.D.D.. You have to aim upwards with this one, obviously.
Alternatively, you can use F.L.U.D.D. -> Cape to get your kill.
1) As Link is coming down, use F.L.U.D.D. (upwards) and spray him as far as you can. If he doesn't go far enough, use your cape. After this, push him even farther with F.L.U.D.D. if possible! You can also jump down, use your cape, then recover... That's just rubbing it in, though. If you're a real jerk/pro, you can hit him with your cape, use a non-charged F.L.U.D.D., THEN recover. Why? Because you can.
It all comes down to knowing how the moves work. Once you have a good feel for both moves, you'll know which move to use in a certain situation and which order to use them in if you have to use more than one. F.L.U.D.D. is awesome, and having this move AND the cape is unbelievable. You can actually get by using these two moves alone, although I don't recommend it. Rather, interweave F.L.U.D.D. and cape combos into your normal gameplay. You can counter almost every character's recovery using some combination of these two moves.
A final tip in closing... Some times, shooting upward is more useful than shooting sideways or down. You want to knock a person down and to the side, but shooting sideways when a person is over the stage won't give you many hits. Aiming upwards will allow you to hold a person in the air, and this is especially useful when the person wasted his/her third jump over the stage.
Note: This is preliminary information, so some tricks could be more useful than stated and some tricks could be less useful than stated.
BTW, does F.L.U.D.D. push projectiles? (such as capsules & DDD's Waddle Dees?)
Yes. =)