I use to use a tablet laptop before I got my MacBook Pro. I used it to draw, but ultimately I didn't use it for much, hence when I needed to upgrade my portable I got a Mac.
As for the usefulness of a tablet pc, that depends on what you'll use it for. If you draw, it would be great. There are some great art tools out there (Artrage is a great app). But if you plan on using it for 3D, I don't think it's very useful. I use the keyboard a lot and the tablet ergonomics didn't work well with pen to screen + keyboard strikes. Folding the pc to just have the screen showing + hooking up a keyboard did work but it's kind of awkward, though that's your best bet.
You should ask yourself if you really need to tablet functionality, i.e., would you go out and draw with it in the park? In my case I didn't need it as much as I wanted. This comes from a person that uses the Wacom Inuos 3 on my desktop for EVERYTHING. My mouse sees very little action. I even hook it up to my MacBook, because I have a hard time using SketchUp with a mouse now. For the money I'd say get a great laptop. Tablet PC's are still pretty pricey.