I'm in the same boat. I love the practicality of the car, and since I don't commute much to work (10-15 min drive), I think I could live with the poor gas mileage.
If I were you, I wouldn't buy your friend's car, but rather find an RX8 with better condition. A lot of 2004 engines had problems strait from their design, so they issued a TSB to replace a lot of them. So if you find a 2004 RX8 with a warrantied engine, take it as a positive note.
I've found this site to be useful here:
RX-8 Discussion - RX8Club.com
hope it helps and keep us posted on your research