yi is smash your keyboard though just make sure you don't hit your W.
But in all seriousness use your Q to engage especially in groups. Once you've Q'd use your E to get some extra damage in there, if they try to run use your ult to catch up.
Also build him pure crit and maybe 1 or 2 AD items. You don't really need life steal unless you think your W isn't healing you that much.
Oh and also statik shiv, just statikk shiv on yi combined with Q gives you massive aoe capabilities
tl; dr: Yi is great for accidentally picking off an enemy by accident just because they happened to be caught up in the group.