I’ve been looking into this too for Silverstone. It’s 50 euros for the weekend, but extra charges occur if the unit is lost, or returned late etc, so provided it is looked after then its just 50 euros.
Everyone I have spoke to who has been to a GP has said it was invaluable, and makes following the race 100 times easier. According to the website for 2010 you will be able to follow 4 onboard cameras at once, see lap comparisons, live timing, team radio, and of course follow the race as if you were watching on TV.
The only thing I’m slighlty worried about is that I will end up watching the Kangaroo TV more than I actually watch the cars, but I’m sure I can tear myself away from it :P