I've seen him play maybe 50-60 times the past few years. I'm on record on another site saying I think he's one of the 5 best wide players in the world, and I truly believe that.
He has unbelievably quick feet and incredible close control and dribbling skill. He also has a strong, accurate right footed shoot and is very instinctual in finding space off the ball, and attacking the box when the ball is on the opposite wing. He works very hard, he tracks back constantly and he's a willing and skilled passer. He's not a true "winger", but more of a wide forward (generally plays on the left and cuts in, though he can play centrally as well). On a Shakhtar team that had a lot of talent and shared the ball around and worked very well together, he was still the clear best player on the pitch for every match he played the last 2+ seasons - even in the CL.
If it wasn't for internal Brazilian politics (the local clubs pressure the association to not call up players who leave young for eastern Europe) he'd be starting for Brazil over Oscar, Bernard, Ganso, Hulk, etc - he's an absolutely brilliant player. I rate him the same as I rate Bale, I think he's that class of player. If he is available for anything less than 30m GBP that's an absolute robbery.