I think he's overrated, but not one of the worst actors. In all the films I've seen with him (Inception, Catch Me if You Can, The Great Gatsby, a little bit of Titanic) he's always playing a sort of capricious eccentric, so I think he relies on the same persona, in a way.

There was that moment in Inception, however, where he's supposed to be witnessing something that should be absolutely heartbreaking for his character, and he comes across as more annoyed than anything else. That took me out of the movie.