As long as you do your research and remember that you CAN NOT under any circumstances change or alter your reservation. Even if your flight is delayed, they will hold your vehicle, but not cancel the prepaid booking.
Using biddingfortravel.com and betterbidding.com will help you get estimates for your proposals.
For instance, I just won a bid for 8 days at MCO for $12 per day for a compact. I saved at least $100, and was placed with National which is on airport. No shuttle!