There's not much out of the norm in 500 Days, but the charisma of Joseph Gordon-Levitt and Zooey Deschanel are hard to resist. I'm not sold on Zooey as a great actress, but she definitely embodies a particular "girl of your dreams" type really well. We never get to know her character much but Deschanel makes her feel familiar and perpetually out of reach at the same time.
In comparison to the other quasi-indie romantic comedies recently, Nick & Norah's and Adventureland, this is far better. Everything moves nicely along, things aren't too contrived, there are lots of good scenes and emotional touch points, it's humorous, and the way the structure bounces around makes everything feel compelling and (semi-)fresh. This movie may not break new ground but at least it doesn't ruin anything that was previously planted.
Joseph Gordon-Levitt is Heath Ledger, Jr. In your mind's eye, without the benefit of pictures, you would shake your head and go, "No way." But then you watch this movie and you're like "Okay, I see it." One review of the movie mentioned that Gordon-Levitt was a "stone fox" and I guess that's about right? He's a good actor, either way.
On a semi-related note, here's a website called childstarlets.com that has pictures of starlets when they were kids. It's fascinating.