Shaco's viable when mastered and horribly underpowered when not.
He's extremely squishy and fairly item-reliant, but his boxes and Q provide him with some very powerful tools for circumventing his weaknesses. I'm sure you've seen videos/gameplay recaps (or actual Shacos in your games, rare as they may be) in which you saw Shaco escape ridiculously impossible circumstances or noticed him plow through jungle camps while taking very little damage.
Basically, he has one of the best utility-based toolsets in the game. Due to the mastery required for using his Q and W effectively (I would list his ultimate, but the clone is very easily discerned from Shaco, making its "trickery" value very low when used against smart players), you won't see many successful Shaco players in lower levels of gameplay. He requires dedication to master because every person a Shaco plays against will react to him differently— relatively few players will walk directly into a box placed behind them into a gank, but a similarly low number of them will know how to deal with the fact that his Q isn't just stealth; it's a blink, as well. Some enemy teams will wise up to a Shaco in the later stages of the game by allowing their tanks to walk into possible box locations or use 
to remove boxes, but many will end up losing teamfights because one of their carries walked into a box.
TL;DR: Yes, Shaco is viable, but you have to be skilled with his unique kit to take advantage of him.
