I have one. Its a neat toy. Only real problem with it is where do you go to test your car with it? Not many spots on the street i want to go from a standing start to around 120mph. at least for my car. Would be handy if local track did test and tune days and no lights or time slips i guess.
A friend of mine has one and I borrowed it for a weekend last summer, the thing is pretty damm accurate but you have to find a flat quarter mile if you want the times to be the same as what you get at the track.
I find it very usefull to practice lauching, I did a bunch of launches with it and by watching the 60 ft times on it I got better at launching my car.