I am in the rather unique position that I have a friend who owns a technology magazine and I write the odd "Technology in the field" type article for them, and I have seen and used a lot of these tablet PC's as a part of it.
Whilst the technology is great, it is still not quite at the point where you can easily walk arround with a tablet pc resting across your arm like a clipboard. You still need to be at a desk really to use them. The exception are the 'mini' tablets (8" and under in screen size).
Tablet PC's are also useless without good software. Unfortunatly, using many apps etc in a tablet environment is not that much fun. I can easily think of ways to use it for blocking, but unless you have an application which accepts input from a stylus like device, you may as well just use pen and paper.
I must stress though that my use of them 'in the field' has been for short periods of time and I did not spend hours looking for the appropriate software. I also primarily used them for sound and video editing, as well as lighting design using CAD - for those purposes they are great.
I would suggest that you try and find a place that does short term laptop hires (there are a few places that do them) and rent one for a week or two, and have a good play.