I have a Gtech Pro RR, and I've compared it at the drag strip for about 20 runs. It is usually within .1 sec and a couple of miles an hour of the time slip, and the most it was ever off was .2 seconds from the time slip. Some say it is even more accurate than a time slip since most tracks measure the time it takes for you to pass the last 60 feet of track to figure your final speed, where the Gtech actually shows your speed at the end of the track. This is why the Gtech Pro RR always reads a couple of mph faster at the track... unless you're pedallin/sandbaggin/broken/redlinin/shiftin/etc.
I bought it to see if I made a difference when I made mods. It's one thing for it to sound faster or feel faster, but the butt dyno isn't always accurate. But this thing was accurate every time I used.
But once you've found out if your mod made a difference, and you know what you/your car will do in the 1/8 or 1/4 or 0-60 or whatever, it kind of looses it's purpose. It's a good thing for a group of guys to share, pass it around to your friends, let them borrow it, etc. Or if you can't make it to the track, it's a great way to see what your car can do, just do it in a safe place.