One of Yoshis best moves, and his very best method of killing.
Its incredibly quick, his fastest aerial by loads so this is you should all by momentum canceling with. The speed in which is comes out also helps to compensate for predictability.
The trick to this move is finding inventive and unexpected ways to get it in for the kill, as anyone worth your time in Brawl will see obvious attempts to kill with this a mile away and AD it. A nice way to actually counter them ADing it is to follow them down in the air, if you can fall with them as they AD you can just Uair again to punish it.
Here are a few ways I like to supprise the opponant with it...
Firstly, a SH Uair is easy to see but delaying the Uair untill u are JUST above the ground can be a nice way to catch them off guard as most opponants are going to be expecting you to Uair straight away, this is also a good way to kill grounded opponants with the Uair.
The Uair also goes through and hits the opponant through the bottom of the ledge on many stages such as Norfair and Smashville, just make your you can get back to the ledge and don't get trapped under the stage.
A really nice way to supprise with Uair that I just recently picked up is to simply fall off the edge and Uair. The falling Uair can easily catch recovering foes and they will be expecting you to jump before the Uair or go for a Fair or Nair. Actually fall off the ledge and then rising Uair works a treat too.
Baiting an Airdodge using eggs is another great way to get it in, an aerial opponant will instinctively AD a thrown egg allowing you to punish the AD with a Uair, works especially well when they are recovering vertically downwards allowing you to get under and throw an egg vertically up.
Faking to invoke an AD is really good too, jump up or SH as if going for the Uair but instead just FF it and hopefully they will AD allowing you to rising Uair to punish it.