OK, so it isn't like a $300 detail nor is it the quickie $7 with your gas and who cares if it scratches wash. It is instead as many folks have noted a Seattle institution. For $20, you get 20 minutes of joy as your kids watch it and you get to remember all those movies feature car washes when you were a kid.
They use soft brushes so they hopefully won't destroy your finish and for $20, it is a decent job. So, not the very best, but a good value for the money. BTW, the downtown location is the oldest and funnest just because the elephant is so big.
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