After reading through the keynotes, it seems interesting - a nice toy.
I'm not sure who is really going to buy one at first; probably just the mac faithful initially.
Considering myself...my old laptop (overhwhelmingly used for couch-based web surfing) is doing just fine...but if I needed a replacement, maybe this would be good enough; it is certainly cheaper than a new Macbook.
Then again, they are asking $499 for the base model that is basically a glorified/bigger iPhone...decent laptops (if not Macs) are available for that much if you shop around. Doesn't matter for me anyway; I'll figure that out when my laptop eventually (but hopefully not too soon) dies.
Regardless, I think it's too early to tell how successful it will be. Certainly could get some support, but I'm not sure if it will gain mass-market acceptance as quickly as the iPhone did...probably not; could be more of a slow/steady growth.
Could do some neat things, but nothing's striking me quite as much as the iphone did.