Is a quick ****. But unbelievably squishy. AD Yi is very easy to counter, just pop a thornmail and watch him cry, or if you're going to 1v1 him, well, don't.
AP Yi on the other hand. ****ing ANNOYING! Get him low? Np just heals it all back ¬___¬
He Can jump into a fight, deal a bunch of damage to all players and get away safely (multi hit ****) Actually very good both early and late game, and I've seen some very good solo-mid AP yi's lol. His downside is squishy I guess but, with his ulti, movement speed, he can go in, deal nuts damage, sprint out and heal anything he's taken.
Best backdoor champion imo, Pushing nexus, o hello yi taking our turrets D:
try him, you will probably like him.
Btw, if you starve him early game (deny him farm, deny him kills/assist) he is practically useless, ends up being a free kill, but that isn't very easy to do all depends how good you are and what champ you're playing. Same applies to near enough any "carry" champ. They rely on end-game to win. Harass early and you'll be fine... providing you don't get ganked :P
Oh... and err, Tryndamere. Best counter to him, or well, any champ really, OP bastard lol. Stun YI once, he's pretty much dead, 2 stuns in quick succession... dead. Crowd Control is your friend.