I've had a Toshiba 3500 Tablet PC for about a year. I'll assume you know the basics of what a TPC (along with XP Tablet Edition) does.
Pros: electronic versions of all your notes/sketches. Ability to print any file to the "Journal Note Writer" printer which then lets you ink over top of the output. Ability to ink over any TIF file using Journal. Decent but not perfect handwriting recognition - it's very good for words, horrible on non-words like a URL. A third-party program from Xthink called Calculator which lets you do some handwritten math and computes the answers. Being able to ink over a document and come back later and search for handwritten comments.
Cons: Decent but not perfect handwriting recognition. It's just off enough to irritate you. Lack of built-in CD-ROM drive. Don't ever lose your pen.
I use it all day, every day at work. But the most glorious part of the experience was when they put in wireless - at that point, it was even better. I can sit in a meeting and read/write email, access files, etc. and not waste any time. Using the pen is totally silent, where even the quietest keyboard will annoy people in a meeting. I can take meeting minutes and as soon as the meeting is over I can email them to the attendees, and they have them before they get back to their office. Using Office 2003 with Outlook, or Office 2000 with the TPC add-on, you can handwrite emails, calendar items, or use ink in any Office app.
A couple weeks ago I was in a meeting and one manager said we could start working on some files he put in a particular network directory. I found the files, copied them to the work directory, emailed the guy who was going to do the work, he looked at it and emailed me back, and before the meeting was even over I was able to report that we would be done in two days.
That stunned the crowd.
The new Toshibas (M200) include Microsoft OneNote which is tailor-made for TPCs. I had the beta but don't yet have the released version. It's a great organizing tool for inked stuff.