I'm in the same quandry. I obsess over new gadgets for the longest time and then after I give in and buy something, I only use it for a little while before it gets put away. (I just bought a DSLR Nikon camera I'd been agonising over for a year and now it's sitting in a drawer until I go on my next trip.)
Knowing this, I've put off buying the PRS-700 - my latest obsession - because I don't want to spend so much money on something I won't use often. On the other hand, I'm a voracious reader. If I have the time I can read 3-4 books a day although when I'm working it goes down to maybe a book a day. I know, I'm such a nerd!
Anyways, my thinking is that if I can find enough ebooks to keep me occupied for the next 6 months, I'm going to take the plunge and get the Reader. I've already managed to load up about 100 free books and have wishlisted a whole bunch on vendor sites so I think I might be able to justify the cost. In the meantime, I've given up buying pbooks and will just re-read the 1000+ books I've got sitting at home.