I have a Mercury Grand Marquis and on a road trip in December, I went to a drive-in movie theatre in Texas after five hours of driving, and decided to stretch out in the back seat during intermission. It was comfortable! Granted I couldn't really s-t-r-e-t-c-h, (I'm 5'5") but I reached from door to door without much bending my knees, it would certainly do in a pinch, providing I didn't store my junk in the back seat.

The front seat is also comfortable, but I'm not sure how far the seats go back. I don't think they go back as far as our Ford Escort which goes to almost flat.

Like the others, I wouldn't suggest sleeping in your car every night. I recommend bringing a tent, just in case. I like dome tents because they are easy to set up without assistance.