Speaking as a male who was surprised how much he enjoyed Juno, I was equally surprised how much 500 days just washed over me. It's a reasonable but unremarkable film, though a lot of people have adored it. The terrible song over the opening credits meant it didn't get off to the greatest start and I just found both lead characters very whiny and and self-centered and unappealing in that very American way.
And for a film which has had so many positive reviews for its real depiction of a relationship, the actual setting seemed no more real to me that the Hanks/Ryan cutesy comedies and their ilk. It reminded me of an American Richard Curtis film actually, right down to the "listing her old boyfrienRAB in the cafe" scene which was a direct lift from "Four Weddings and a Funeral".
The structure was interesting but there's a dreadful final joke and look into camera turns the whole thing into something resembling a shaggy dog story. If it hadn't got such glowing reviews, I'm not sure I would have been so disappointed in it.