After using it for a day I'm a little torn to be honest. The double amount of ram clearly makes the biggest difference for me, you notice it right away and it's so great not to have so shut down safari all the time just because it eats ram. The haswell processor and faster ssd of course makes the Air quite a bit snappier, but I really don't tax it much with my use so I don't notice it as much as the ram. The background lit keyboard is nice, but really doesn't make much of a difference, I never really missed it on my 2010 . That's basically it for the pro's. The screen is on par with the old model, a bit more yellowish but that might be the glue and might get better after a while. The speakers are actually better on the old model, these new ones aren't exactly bad, but the old one has a much clearer sound which is quite surprising. The better battery was never a big deal for me, I was happy with the 7 hours on my old Air, but it doesn't exactly hurt to get a few more hous out of it, I just see it as more of a bonus than a real reason to upgrade.
All in all, I think I'm going to keep it, mainly because of the extra ram. If you're looking at the base model with only 4gb I would advice you to think twice about it before upgrading, try it out in a store first if you can. I only really upgraded now because I didn't want to miss out on the 100 dollar iTunes gift card from the Back to School-campaign, otherwise I would have waited for the release of Mavericks to see how it would run on the 2010 model, I think I've read about Safari handling memory usage differently among other things. Hopefully it will be released soon so if you can hold out, wait for it and see how it affects your performance.
Dont get me wrong, the 2013 Air really is a great computer, it just feels a little underwhelming when the exterior and the screen are exactly the same as they were three years ago. While it's still okay, the screen could really do with an upgrade, and hopefully it will get one sooner than later.