I bought the groupon, and I was getting close to the expiration. 4 days away to be exact. I called on the 9th, for a groupon that is going to expire on the 15th. I was a bit concerned about getting an appointment prior to the expiration due to the fact that spring has just sprung and everyone wants a clean car. The guys were extremely nice to deal with and I was able to get an appointment within two business days.
I dropped my car off at night and just dropped my keys in the drop. My car was all finished up around 3:15 on the day of the appointment. I got exactly what I paid for and couldn't be happier. My car wasn't a complete mess, but needed a nice touchup. The one thing that I was most pleased with was the windows. Inside and out they looked great.
You do have to pay the tax on the detail upon pickup. I'm not sure about the technicalities of this situation, so I don't feel justified to take away from a good experience to something I don't know the fine print on. And I'm not sure if you are supposed to tip on a car detail, but facebook land seems to say yes. So I guess that's that.