I've seen even the currently-crappy tablets in heavy use in the medical field, I'd bet largely for signatures (I've signed on 5 separate tablet PCs at different locations, over several years). They're also capable of being carried around and used while walking.
That said, they are still crappy, and most require a powered pen (won't anybody learn from Wacom? Or did they somehow gain a ubiquitous patent, which the patent office isn't supposed to allow?). At the end of the day, though, they're just Windows with a couple marginally-intelligent tablet-oriented applications (ie, big buttons). Certainly not optimal.
Apple is attempting to take the lead here, defining what a tablet should and should not do. If it works out, then I suspect other tablet manufacturers will follow suit and create devices with similar scope to the iPad. If it doesn't then it's back to the drawing board.