Never used them but have seen older threads on here or you can search around on-line for people commenting on their experience. You are basically renting a car from a private party and some of the things that can go wrong is if the person doesn't show-up when/where you agree then you are stuck with no transportation. When returning the vehicle if at the airport and they don't show up, do you want to risk missing your flight? Others have commented about endlessly haggling with the person when they return the car about excessive 'wear/tear'. I would hope they have the right type of car insurance since when someone uses their vehicle as part of a business it requires different coverage (similar to those on-line taxi companies), but likely you will never know that for sure until you have an accident. So if all goes well, you might save a few dollars, if not are you willing to take that risk?
I would personally never let a stranger use my car, but apparently it works for some to make a few $$$ on the side giving out their vehicle.