I just read a good review, but I don't remember where.
The idea was that while tablet PC's look really cool, and are a neat concept, are they worth the extra price? The reviewer wrote about how you still have to plug in a keyboard, unless you want to deal with the ergonomics of typing on an "on-screen" keyboard.
If you're not worried about typing, or don't type a lot, it may be a great alternative for you.
I apologize for not knowing more about them.