Well, check out MS's site. They have a prog called OneNote, which may be the killer app. It lets you integrate writing, speech, pictures, typing, etc in a free-style layout. The tablet costs a lot more than a PDA or laptop though, so watch the budget.
PDAs are good choices for long battery life, portability, and price. Writing on the screen is much less fun (or useful), but having a folding keyboard and PDA will help a lot with text-oriented notes and with other tasks (You can watch movies, play 3d games, and listen to music on PDAs).