i prefer to use a pressure washer first and then do a hand wash after, only because i dont like to rub in the grit and dirt that builds up on the paintwork by using a sponge. i just use the pressure washer to remove the top dirt, i wouldnt concentrate the blast to try and remove stubborn dirt, use a sponge for that
as long as you take care i dont see a problem, its clear that they could damage your car if you are reckless with it, but otherwise its fine for me!
