I'm upgrading the Operating System on a friend-of-a-friend's tablet PC right now. So far, I hate it.
Here are my complaints:
I HATE using the stylus as a mouse. I am good with mice, trackballs, pointing-sticks, touchpads, etc. But this is a real learning curve. My phone is a Palm Pilot Treo 650, and I'm good with the stylus on that. Also, the handwriting recognition on my phone is much better than what comes with XP Tablet 2005.
The screen is too small, but more importantly, they don't come in wide-screen format yet. Wide-screen is the way of the future, and I'll never go backwards from there. I always figured that when they did start making them wide-screen, I'd ditch my 17" WS notebook for a 14" WS tablet. Now that I've played with one, however, I'm really not too likely to do it.
A lot of the software I like to install people doesn't like the Tablet OS. They have odd/unpredictable crashes. But the crashes are so random, that you still feel like trying to get by in the hopes that it won't happen too often.
The bottom line is that I just don't think it is a fully developed solution yet.
Check out this guy's experience if you're really serious about this, though. Even though this material is pretty dated (until the last entry), I think it covers most of the ground you'll need it to.
http://www.pdantic.com/reviews/tabletdiaries1.htm