Low replacement cost is a really good feature for a mobile and helps me appreciate the small size and weight more. For example, if I go somewhere with my MBP it stays with me at all times. That's a huge pain in the ass sometimes, the last time I was in Vegas I had the MBP on my shoulder for 4 days straight. But if I go somewhere with my Aspire One I don't feel bad about leaving it in the glove box or whatever.
I wouldn't make a netbook my main computer until they get a bit more power (especially VRAM), but it makes a great accessory. I have used it a lot more than I thought I would. It's quite popular when I have people over too, which is nice because I don't have to worry about what's running on my main computer (shell sessions and what not). If someone wants to drink and YouTube there's now a laptop sitting on the coffee table at all times.
Probably not for everyone, but a very high percentage of people who have used mine expressed interest in getting one. Wouldn't be surprised if a few already bought one.
I own a Mac Book Pro and bought an Aspire One primarily because I was tired of lugging around the MBP. There is a huge difference. It's hard to realize until you have both, but the MBP feels giant and I hardly move it now.
One thing I bet they will do in future revs is up the video power so you can hook it into larger external screens--the VGA out to my 24" doesn't look so hot.