I have a G-tech pro and it was worth it. I got it for free awhile ago on a deal from Tire Rack They are high on MPH (or track is low, since track MPH is avg over last like 60 ft), and are relatively accurate. They may measure a little fast/slow, but are very consistent and can be used to judge changes to your car pretty accurately. Important things are to use as level a road as possible, go both directions on the same stretch, make multiple runs, and average them. It might even be worth buying, or the new competition one, if you get a good deal or you can't get to a track easily. I kind of like mine, since I've never been to the track
I have a G-tech pro and it was worth it. I got it for free awhile ago on a deal from Tire Rack They are high on MPH (or track is low, since track MPH is avg over last like 60 ft), and are relatively accurate. They may measure a little fast/slow, but are very consistent and can be used to judge changes to your car pretty accurately. Important things are to use as level a road as possible, go both directions on the same stretch, make multiple runs, and average them. It might even be worth buying, or the new competition one, if you get a good deal or you can't get to a track easily. I kind of like mine, since I've never been to the track
I have one that I bought off of a friend. He sold it to me for $40! He just didnt want anymore!! But its a great thing to buy. As blkZ28 was saying, make several runs and find a level road. Make sure that you have the curb weight of the car including your weight added to it as close as possible. I recommend it. You wont regret buying it!