(Disclaimer: Xaile is not an expert Lucario player.)
Do I use Double Team?
Not very often...
The main reason I use it so sparingly is because it's so SLOW to come out. Back in Melee I played Roy for the longest time and I
loved his counter, because it came out when I hit down-B.
I can't tell you how many times I've hit down-B, HEARD the sound-effects for double team go off, and get hit with their attack before the counter-frames even start.
Sure, Double Team can Kill, but you have to consider a couple things. If you're at a high enough percent to have enough Aura Boost to kill them, can you really afford to potentially lose a stock by mistiming or bad prediction? When Lucario is at high percents, I usually try to make a point to play safer and make sure they don't land those crucial smashes and other attacks that can reset the Aura Boost. Double Team seems like a step in the wrong direction.
That said, Double Team has it's uses. Sometimes people are just really predictable, spammy, or whatnot, and you can bank on hitting with it.
But, the risk versus the reward just doesn't seem worth it, considering all the other options you could do whenever you would Double Team.
(It's really the start-up time that I don't like on Double Team; I miss Roy's Counter.
)