There is simply nothing bad I can say about this movie. It surpassed all of my expectations. You are told upfront that this movie is NOT a loves story, but rather the growth of the relationship between Tom and Summer, played perfectly by Joseph Gordon-Levitt and Zooey Deschanel. The movie is not done in a straight-forward way, but actually jumps through 500 days and shows the comparisons to some situations to show how they've changed from Day 1 to Day 500.
Although it is a limited release movie, I suggest you go out and find the closest theatre that plays it, because (500) Days of Summer is totally worth the drive...and more.