If you plan on taking the car to the track often, this is a very inexpensive solution to adding pre-load to the rear end. Very helpful for optimizing traction with slicks and or sticky tires. Will also help if you run around on sticky tires on the street, hooking up that is. As for the street, if you plan on running large tires on rims that have odd back spacing sizes that would cause the tires to rub the fender trim or lips when you hit a bump, air bags will help here as well. Another benefit is, if you remove the front sway bar for weight transfer, the dual air bags can be pumped up to 10-15psi in both for street cruising. Doing so helps eliminate some of the sway. It won't cure it 100%, but it sure beats not having anything done to compensate. IMHO, I would recommend running two.
I've found my ideal pressure settings at the track to be, 27psi in passengers side bag and 7psi in drivers side bag. Then before heading home, I would equalize them both to 10psi and ride.
HTH
Patrick