I bought one way back in 1995, I think when they first came out. Definitely has repeatable results, but setup can be a pain (getting it perfectly leveled, and inputting the exact correct weight of vehicle with fluids and driver weight). Comparing it to real-world dyno results, it seemed to be pretty close. An engine that dynoed at 139 hp showed 121 to the wheels with the G-tech.
The 0-60 and 0-100 results were pretty close, and very repeatable. The only thing that was off was the 1/4 mile times. It showed slower ET's and higher mph compared to my real results.
One neat feature was the lateral "peak" g-forces. I used to have a favorite on-ramp I loved taking my Ascona on. I hit a peak of .93 G's with snow tires! Also hit a peak of 1.31 G's on a road racing course with slicks. I used to LOVE strapping a passenger into my car and watching the G-forces break 1 G on DOT rubber, at 4 to 5 times the on-ramp speed limit. Ah, the ignorance of youth......
Bob