Despite a few drawbacks that are common in practically every movie (moral ambiguity, incomplete characterization for female roles), this one is very classy and refreshing. It's a romantic comedy, but there are no obvious gimmicks thrown in just for laughs. There is a wedding scene, but that can be excused since nobody crashes the ceremony, has sex with one of the bridesmaids, spills anything, or dances like a drunken fool.
(500) Days of Summer is about a guy's attraction to a girl in his office named Summer. The guy has feelings of true love for her, and is committed to her, but Summer won't commit to him. She likes him enough, and for all intents and purposes they are a couple, but she'll never say that out loud. And that bugs him. Bad.
We both recommend this movie. It is a well-told story with a different, catchy sort of feel to it. And there is a happy ending, just not the one you might think.