Matt Hurt’s review published on Letterboxd :
(500) Days of Summer admits upfront that it's not a love story before taking the viewer on a journey through hopeless romantic Tom's search for true love. This journey is through his relationship with Summer which, to the outside observer, is problematic but to Tom is his one true love.
I've yet to see a movie that paints as wonderfully honest a picture of a hopeless romantic when in pursuit of who he thinks is his soul mate as (500) Days of Summer does. Over the years, I've watched this movie several times and find myself identifying with Tom in a lot of different ways. And the movie's nonlinear structure helps tie the story together and puts unique emphasis on certain aspects of the relationship and its counterpoint post breakup.
The movie embraces some of the tropes of its genre (having kind of one-dimensional best friend supporting characters, public outburst of emotional monologue by main character) but it also successfully breaks some ground for the genre. There's an incredible sequence scored to Hero by Regina Spektor, wherein Tom attends a party. It's a split screen with one side showing his expectations for the party and the other side showing reality. The sequence is so perfectly shot and timed that the end of that sequence alone hits me hard every time.
The movie has one of the best endings of any romcom I've ever seen, for a couple of reasons. One, the use of "She's Got you High" by Mumm-Ra is absolutely the perfect song to end the film on and play over the end credits. More importantly, though, that last scene is the perfect button for the movie because it doesn't give us a Hollywood ending. It eschews the traditional romcom ending in favor of a slightly ambiguous, cheeky, and nearly fourth wall breaking almost wink at the camera. It phenomenal because it leaves it to us to discern what Tom's life post-Summer life will be like.
My favorite thing about the movie, though, is how it depicts Tom as a lovelorn, hopeless romantic, bitter dumpee, and uninspired in his professional life. But the movie is about how his experience with and without Summer shapes him into a better version of himself. Which I feel is a much stronger arc than a more clichéd romcom would have had.