I think people treat him a some sort of world beater, I don't think he is...he's very good but he's not top bracket for me and will never hit the heights of the very best strikers in the world.
At West Ham he ran around a lot and worked hard but went a long period without goals when he first came, he then got some goals at the tail end of his stay there. At Man U, I'm not sure he totally convinced Ferguson, if I remember correctly (correct me if I'm wrong), he was on the bench a fair amount rather than a starter.
Was great for City in his first season, what I will say though is that he doesn't seem as explosive as someone like Aguero, I think he's easier to stop and less likely to score a goal out of nowhere.