We rented with Dollar a few years back (2006 or 2007?) and also rented a car seat. When we arrived, the guy went back into this little room and picked a seat from a bunch. ALL the seats in the room were plastic covered. When he gave it to us, it looked brand new, never used. Now granted, we've never rented a car seat since then so maybe our new seat has been used and abused by now! But I wouldn't hesitate to rent one again.
And if I remember correctly, they didn't rent them on the website. I had to call and have it added to the reservation. I believe back then it was $9.95/day.