I bought one, then freaked out the first time I used it on black paint. Now it's used only to clean the shower door each morning.
Saw a guy using one on a classic Ferrari on some Discovery show.
Use it on glass only. Also, to aid drying, you know in the final rinse to take off the hose nozzle and just let the water flow freely out of the hose? That causes a bunch of water to slide off the car before you even touch it with a towel.