Actually, I haven't invested in one. Yet. From observation, however, the Tablet PC seems idea for corporate environments especially if one travels and do a lot of meetings. Aside from handwriting capability, its mobile, flexible, lightweight, and good for reading. One's needs would determine which product is better suited.
I only know of three -Compaq, Gateway, and Toshiba. From what I can tell, they all have their high and/or low points. Compaq has good video graphics, and is a lighter weight. Toshiba is a faster processor and has a bigger brighter screen, [better for presentations] and has a higher battery life (6.5 hrs) than Compaq. Gateway has a large hard drive (40GB) and other accessories. My focus and preference is Compaq because I like Compaq computers. However, I hear Toshiba's Tablet PC is really good.
I think Tablet PC's are worth the price, as they function as the laptops and desktops but are much more mobile. I'm seriously thinking about purchasing one.