The story's pretty simple. Tom is a typical hipster (I'm sorry, I don't know any other words for intelligent, snappy dressers) with a mediocre job that he's better at than his friends. He believes in true love and he likes The Smiths. Summer is every hipster boy's fantasy girl (conveniently played by every hipster boy's fantasy girl) who doesn't believe in true love and likes The Smiths. The film pretty faithfully follows the inevitable tale set up by these parameters.
Some time-hopping is done (we see the '(500)' flip from day 13 to 343, back to 14, etc.) and works well early on as a way to starkly contrast the emotional roller-coaster that is any affair, but eventually falls into gimmick territory. This isn't "Amores Perros" and it's not "Memento," it's a modern version of a classic idea, and it's entertaining enough not to rely on so many bright flashy things (probably a symptom of director Mark Webb's CV being made up almost entirely of music videos).
Still, the charm, the unique quality that is simply a more realistic view of a love affair (including a recognition that the characters themselves do not view their affair in any light close to realism), the bluntness with which we are made to ride the roller coaster with the protagonists, it all overcomes the issues I'm nitpicking at. We're more 'in' the relationship with these two than anyone has ever been while watching Kate Hudson. It's a romantic comedy for those of us who are a little too cool for romantic comedies. If you roll your eyes at "Letters to Juliet," you may find the ways that Summer and Tom embarrass themselves - thus becoming more attached - a little more realistic and entertaining.
Some final supporting roles notes: while Chloe Moretz, as Tom's sister, is playing almost the same character as she does in "Kick-Ass" (grown-up in child's body, minus the murder), she is still a scene-stealer. She's so good at what she's doing, it's worth noting here as continued prediction of her great success.
And fans of good television will recognize Clark Gregg, who plays Tom and Summer's boss in "500 Days" from his role as the mysterious stranger in the bar at the end of the second season of Sports Night. I've seen him pop up in a few other things since then, obviously, and he's always proficient enough to warrant my wishing he got more attention and bigger parts.
The bottom line? If you already think Zooey is precious or twee, your head will explode while watching "500 Days." If you like retro dresses, guys in vests looking disheveled, and have ever been crushed to a pulp by love, then you will have a dandy time watching this film.