Lots of great (and more comprehensive) reviews out there already so this just focuses on my initial reaction.
All in all a pretty amazing device that I would describe as an iTouch on steroids. This is not a desktop device (like an iMac or a Macbook for that matter). This device is something best used sitting on the couch, a comfortable chair, or even better, on the floor.
I had some initial concern that the flat design would be awkward to use. It is compared to a traditional notebook but if your frame of reference is an iPhone or iTouch it is completely natural.
Surfing the Internet, looking at pictures, reading a newspaper, watching a movie is a fantastic experience. Reading email is fine but typing longer emails will take some time to get use to (a few days, I suspect).
Bottom Line. This is probably the ultimate media consumption device but it's not a true 'content creation' device (yet). If you use your laptop primarily for Internet, Email, News, Video, Pictures, etc, this can be your primary device and the 10 hours of stated battery life is game changing. If you need content creation tools (Word, Powerpoint, Excel, etc) for business, this is a great secondary device but don't toss the laptop.