A couple of years I got a Wacom tablet. It was a gift – not really the type of thing I think I would've bought, but it was definitely nice to get. Since then, I've found it incredibly useful, and it's become almost a permanent fixture in my bag.
For a start, it's a great way of being able to ink up documents in ways that I can't do with my regular laptop (I don't own a Tablet PC, but inking is still useful from time to time).
But I also find that it's really useful when I'm teaching or doing any kind of presentation. There are often times when I want to draw some sort of diagram. Having a diagram pre-canned can be useful, but it's great to draw a diagram in front of the audience. Something about them watch you construct it helps them. If I have a regular whiteboard, then I'll use that, but if it's a diagram that I want to be able to refer back to, then I either have to find a part of the whiteboard which I don't want to use again, or else I can pull out my Wacom tablet and pull up Paint.Net. The pen is touch-sensitive, so it draws a grey line if I'm drawing lightly and a black one if I'm drawing heavier. And I can always switch back to one I drew earlier, and add to it, correct it, whatever. I can even email the diagram to students who want it (but that's not something I do regularly). It also goes really nicely with ZoomIt, which I use all the time to point out the detail in screens.
I do walk around a fair bit when I'm teaching though, and I notice it when I'm restricted (like when I'm using my Wacom tablet via a USB cable). My plan is to one day get one of the Bluetooth ones that Wacom have, and see if it's different. I won't be able to use it in aeroplanes, but I can imagine passing it to those students who don't want to get up to write on the whiteboard (which I do from time to time in my classes).