[Yetiman]

I have one, and it is a pretty useful tool.

While it isn't a substitute for a dyno, what it has done for me is allowed me to work on my launching technique and get a lot faster than I was at 0-60 and 60 foot times etc.
Thats pretty much all I have used mine for anyways, and it was worth buying for the improvement I saw there.
It also gives me a baseline to see if changes to the car, tuning, tires parts or otherwise are improving the performance or not.

I am lucky enough to have an isolated frontage road very near my house that gives me a somewhat controlled place for comparisons.