Great value car wash. You'll end up paying pretty much the same amount if you go to an automated car wash or one of those pay-to-spray car washes. Plus it'll take you two or three times as much time.
I've always had good experiences with the workers. They're courteous and do a good job if you have the right expectation. This is not a place to get your car detailed, but also if you want that level of cleaning you should expect to pay a lot more. At Mermaid, they not only wash the outside but also vacuum, wipe the inside of the windows, and dry the outside. I don't think they wipe the non-window area of the car (maybe if you pay more).
The coupon system is a little weird and not very practical. They used to honor old coupons but not anymore. Basically you can save a buck or two if you wash your car every 3 weeks, which doesn't seem practical for most people.
Overall a good value for what it is.