The movie is about the ascent and decline of a couple's relationship. The 500 Days is actually rather misleading - Day 1 is when the couple meets, they start dating around Day 80 (I think?) and break up no later than Day 300. The next 200 days seemingly refute the truism that the amount of time it takes to get over the end of a relationship is equal to approximately half the time the relationship lasted, focusing on the mostly uninteresting misery of the young man.
The movie attempts to be clever by jumping around the chronology, contrasting scenes of them first getting together with moments where things begin to fall apart. This is occasionally interesting, playing with echoes between those scenes and showing how similar events can be inflected in completely opposing moods. What is distinctly missing here, however, is any sort of play on perspective. The movie is obviously related from the guy's perspective, but coupled with a voice-over from an omniscient narrator, which belies its entrenchment in the guy's worldview. It might have been more interesting if it showed how his memories weren't entirely reliable, or changed over time. Alas, it did not.
The major problem, though, is that the characters are totally flat. You never have any sense of why they love each other, why they cease to love each other (or if they actually do), or really what they're like as people. Their characters are conveyed through standard hipster cliche. They're walking stereotypes. They go on a date to Ikea - you can pretty much write the dialogue yourself, just based on that. Which isn't to say it isn't kind of cute, but it doesn't really tell you anything. Particularly obnoxious to me was the fact that they provide you with all the usual cliches of falling in love, but when it comes to the deterioration of the relationship, the movie develops cold feet. It can't seem to portray either of them as actually doing anything wrong, or disliking each other. The contrast that comes to mind here is Eternal Sunshine of the Spotless Mind, which likewise portrays a relationship, even in a similar way, but which dares to have its characters occasionally act like complete jerks. And really, this is how the movie could be summarized - it's like Eternal Sunshine, except without all the stuff that made that movie so good. It just doesn't really have anything new to offer - it's the same old thing, so much so that it doesn't even bother to create unique characters or situations, it just recycles cultural tropes that may be relatively new, but already feel stale.