I bought a pair of the new iPod in-ear headphones today at the Emeryville, CA Apple store. My impressions so far...
They do offer noticeably increased bass response over the standard ear buds. However, it's not as dramatic as I was hoping. If you're not familiar with in-ear style headphones, you have to make sure they're inserted into your ear properly to get optimal sound. They need to fit snugly in your ear canal. Apple includes 3 sizes of rubber tips so that you can get a good fit. It takes a bit of practice and adjustment to get them in properly. Once they're in, I find them quite comfortable.
The overall sound quality (to my ears) is quite good... though I'm sure audiophiles won't be very impressed. The real benefit of these headphones is that they block out external noise very well, and they don't leak much sound at all. This will be especially useful for those of us that like to listen to music at night, but share a bed with a light sleeper. I have a pair of Koss PortaPros which I absolutely love, but when I use them, everyone can hear what I'm listening to (no, I don't have the volume up too loud).
I would say if you're happy with the headphones you are currently using, definately try to listen to a pair of these before you upgrade... you may find that it's not worth it. But if blocking out external noise (or not disturbing those around you) is important, they're a decent (and affordable) upgrade. I'm glad I bought them.
I'll be giving them a work-out on Monday when I fly from San Francisco to London.