I have one and like it. They give instructions for runs for estimating HP/Tq etc., and I did 4 runs like that prior to getting a canned SCT (no mods on my engine yet--32V SVT--I just wanted a baseline). After the tune, the car just felt like it had no wind, whereas before I had to be careful not to blow past redline. I put the G-Tech on again and did 3 more runs and it was showing peak HP down almost 25%. To make a long story short, the car's a 99 but the engine is a '98 (came from the factory that way). A tune for the 99 was loaded. The IMRCs on the 99 are different apparently in just more than materials because there's no enable/disable option in the tune for that year. Once the tune for the 98 was loaded and the enable IMRCs checked, it was like .... Whoa! I put the G-tech on again and on my last set of runs, it was showing about 12% increase over my first runs.
So, were those numbers dead accurate? Probably not, but I don't care. I just wanted something that would indicate whether I was making progress or not and it certainly confirmed my subjective impressions. I'd recommend it. As someone said, it's sure a lot cheaper than a dyno for casual stuff (and fun too).