500 DAYS OF SUMMER was watchable and occasionally amusing, but overall it was a pretty big letdown after all the hype I had heard. I suspect that 500 days from now I won't even remember if I saw it.
I also suspect that how much you like this film is directly proportional to how much you like Zooey Deschanel. She certainly is cute, but as an actress, I find her pretty one-dimensional, and the screenwriters don't really make much of an attempt to make her character very sympathetic, nor do they make much of an attempt to explain her. At the end of the movie she is still an enigma, which is very unsatisfying to the viewer.
The screenplay, on the whole, struck me as being written by someone who was either very young (like 20) or who has never been a serious relationship. The whole film seemed very juvenile. I hate to say "film-schoolish" because it was better than that, but it is a deeply flawed film with a few brilliant sequences. The two writers went on to do The Pink Panther 2, which I think says an awful lot about them.
I'm sure there are plenty of people out there who will enjoy this, though.