I've had one for about three weeks (Toshiba). I'm evaluating whether it's a good idea for other areas of the company to use Tablet PCs instead of conventional laptops. Since the Toshiba is one of the convertible TPCs, it would make a nice laptop even without the pen features.
From a spec point of view, the Toshiba has the largest screen and the fastest processor of the TPCs. But battery life runs about 3.5 hours and other brands with smaller screens and/or slower processors have more battery life. For what we're looking at, we needed the large screen.
I've used the Compaq a little and the "feel" of writing on the screen is much nicer with the Toshiba. The Compaq pen has a tip that retracts a tiny bit which I found annoying, and the feel was like writing on sandpaper. The Toshiba has a very smooth feel and the larger screen is nicer.
The handwriting recognition is pretty amazing but it also has a few annoyances. It always inserts a space after converting some handwriting which isn't always desirable. On the other hand, it's funny to watch it try to convert anything you write, even if it's meaningless squiggles.
One of the nicest features of the handwriting in the Windows Journal application is being able to sketch a diagram or something in the midst of the writing. Can't do that if you're typing everything.