Let me start off by saying, I love this champion. Going top lane is meh for me, mid is pretty good, but jungling is where he shines IMO. Keep in mind I'm level 16 and absolutely fail at this game, so what I'm about to say may make no sense at all :P.
His Q: It's a pretty decent skill I guess, but not worth the energy costs. This champion eats through energy super fast, so something that does very little damage that uses alot of his energy? I usually get it once at level 2 and max it last.
His W: This is his signature move, and I like it, but either increase the range or decrease the cooldown. The cooldown is insane, and it's a big reason why I max it before Q. Learning to time his shadows is kinda hard sometimes, but if you get it right, slow them, ult, then unload... It's great.
His E: Great for jungling. Seriously, it's great. I get it at level one, and max it first. The slow is short but it's useful.
His R: Remember what I said at the beginning, this post may make no sense whatsoever. I think you should either give his ult a base damage, or make it deal true damage. I mean he is an assassin, right? It's not like true damage ults are unheard of *cough*Darius*cough**cough*.
Ganks: The ganks are great if your ally has some form of hard CC. I've noticed all I manage to do is drive them out of the lane if we have no CC.
So, yeah. I think he could use some minor tweaking, but he is in a good spot right now.
EDIT: His move speed is too low for someone supposed to run in, stab someone in the back, and GTFO.