Any list of overrated players has to be heavy with Brasilian & English players
Adriano, Robinho, El Shaarawy, Hulk, Balotelli, Oscar...and a litany of English players too numerous to list (Start with Jack Wilshere)
Robinho was a bit underrated for a while I thought, was Madrids only threat for some time but wasn't really taken seriously...
Oscar was a pretty good player I thought, but maybe very overrated by some yes.
I think the case with most players over/underrated is that it's done by the same sort of people mistaking some form or even a good run of luck, looking at the players age and making conclusions. Then when they lose form/luck they are suddenly terrible.
Bojan is a nice case of a player both over and under rated.