I am surprised by the great ratings ACS gets. I went there because of them. However, I rate my experience as bad.
My transponder had been working fine, it just needed a certification. It got flaky once they started sending signals with their test equipment. Maybe removing it to read the serial number caused something in the old transponder to go (not their fault). They had some issues with it but still certified it. I felt they should have failed it, but it was late and I needed to go. His whole procedure didn't inspire much confidence in me. Finally he gets it to pass. On taking off, the light didn't blink anymore and ATC wasn't getting a Mode C reading.
Also, I had made an appointment for the transponder check (and IFR cert) maybe a week earlier. When I got there at the appointed time, they guy's brother informs me the guy went to fly someone in a King Air and wasn't sure when he'd be back. I did not get a phone call or other indication about this ahead of time. This was on a Friday, so I had taken time off work to get it done. Fortunately my airplane partner is building an RV6A at Taylor, so I went to hang out at his hangar. Three hours later the guy shows up. He no longer has time for the IFR cert, but can do the VFR transponder check.
I also tried to get an estimate from them on getting a radio installed. I couldn't get a straight answer on what it would cost. They just couldn't come up with a firm estimate of how long it would take. This was for a single VAL INS 429 indicator on an almost empty Cessna 172 panel. I already had a NAV antenna installed. They had to hook up power and ground, connect the antenna bayonet connector, and hook the audio output to my audio panel. I already had a harness with the wiring. It was not so much what they wanted to charge, but rather their uncertainty that bothered me the most. They ended up estimating 8 hours of time, but weren't sure. They also had to get back to me on whether I would have to send the radio back to the manufacturer to get a form 8130. They weren't sure about the requirement for that either. It seems to me a radio shop should have been more certain about these subjects. I called VAL, and they didn't think it was necessary, though they'd be happy to look over my unit.
I called McKee and over the phone he quoted me somewhere around 2 hours to install the radio. Took the plane down, confirmed the estimate and was done within the estimated time. He was also able to troubleshoot the transponder issue and then sent it in for repair. ACS had mentioned if I had issues wth mine they had some nice Garmin GTX 327 that I should consider buying since old transponders weren't worth fixing.