If anything, it's underrated, what with the whole "It's a Disney game, so it's for kids" thing.
And the only overrated game in the series, IMO, is KH2. Sure, it's fun, but it's too fillery to keep yourself from getting bored every couple of cutscenes. Well, only in the Disney worlds, really, but you get my point.
Birth by Sleep might also be a bit overrated, but it's an epic game, so who cares? >_>
Official Luxord of the Kingdom Hearts: Birth By Sleep Board