I'm on my tablet right now =].
My school required everyone in my class to get a tablet PC for school. Mine is a bit heavier, 14" ~7 pounds, but with a sweet graphics card and more power than th smaller ones. I've gotten used to it, and use the tablet function occasionally.
It really depends on what you do with it. On the go, it's great, way better than a trackpad (I use a mouse, but some don't). It is great for taking notes, a lot better for showing relationships between items. Tablet support in OS's is good. Mine came with XP tablet edition, pretty good. I'm actually using Linux now (Ubuntu), but I still got it working fine. Make sure it is a Wacom.
Tablets are really useful in some areas. In other, the added cost isn't worth it. For smaller computers, I think a tablet is great, an easy way to use the computer. On a desktop replacement, not so much.
Overall, if you can afford the extra money, go for it.