I wonder how long it will take Apple to ship more iPads than the tablet PC has shipped since being conceived. I think I was the first person to buy one at the store I purchased it from. And I haven't encountered another person who has owned one in my day to day life.
I purchased a Tablet PC about 6 years ago to use for school. It was heavy, hot, slow, and the battery sucked. I had to carry an external battery around just to make it through two classes. It was clunky to use, the only useful app being OneNote. 45 -55 minutes of life, and that was after installing aftermarket software to lower the clock and voltage settings of the CPU. Oh, and the hinge cracked and broke within 18 months.
Having used the wife's iPad for about a month, I can tell that this thing would have been far more useful, paired with a small bluetooth keyboard, than my tablet PC ever was, and it costs about a third of the price. Even without a keyboard, for jotting notes, it's good enough.