We've used QS twice. Last year for our first QS rental, and just the other week. Both times, we bypassed the Terminal Counter, and headed to the QS booth, and both times were smooth and took maybe 5 minutes.
We flew into A terminal (Not sure if A & b terminals use the same garage), and here's exactly how to get to the QS booth:
After getting luggage at baggage claim at Level 2, head down to Level 1.
Right around where the in-terminal
Alamo counter is, exit the terminal, and look for the Alamo signs to the parking garage.
Cross the street to the parking garage.
Either take the escalator or elevator to the 2nd. floor of the parking garage.
If taking the escalator, turn right to the QS booth.
If taking the elevator, turn left.
The QS booth is about 50 feet or so from there (you'll be able to see it).
Unless it's changed, all first time QS rentals should be able to take care of any final membership paperwork at the QS booth.
(You have to stop there anyway and provide them with a credit card and driver's license)
And both times we used a coupon.
You can't use a coupon for on-line check in, as pointed out in an earlier reply.
Hope this helps,
-Mark