500 Days of Summer is what happens when life puts you in a situation you're not ready to handle, with people of opposite interests.
The good. Excellent editing. Great timeline handling. Very interesting characters. Nice moody story. Realistic events, actions, and consequences. Solid dialogs.
The actors. The obsessive love sick puppy portrayed by Joseph Gordon-Levitt could hardly have been better played. He gives the role a certain innocence and a dose of logic. Zooey Deschanel, who's usual characters are very sweet, plays an indifferent and disabused, almost callous, version of her usual role. And Geoffrey Arend plays the annoying idiot/clueless well-meaning friend pretty well.
The bad. The diverse situations can be uncomfortable to watch at time.
The ugly. Nothing.
The result. A film that will force you to take position.