I have an NEC Versa LitePad tablet pc. It is completely awesome for class (except battery life sucks, ~2.5 hours).
Most class material is available in Powerpoint form, or in other forms online, and it is great to be able to annotate your notes directly on the screen in your own handwriting. I'd rather carry around a 2.2 lb computer, than a stack of printouts. I'm just sure to back up my stuff on my desktop.
I haven't had any problems with the tablet.
I wouldn't buy a $2500 tablet as a desktop replacement though. Way too expensive an option. Instead I use my P3 933 mHz, 20 Gig tablet for portability only! It is a true tablet (not a convertable) where everything is external. Only input I normally have with me is the stylus because keyboard and mouse are separate. I have a P4 3.3 gHz, 160 Gig VAIO desktop with all the goodies for home use. Total cost was about $2100 ($1100-ish for the desktop, $860 tablet pc, both online).