The G-Tech uses an accelerometer and a clock to do it's speed calculations and it's inherently quite accurate. That being said there is a couple of things you need to keep in mind.
If you are looking at 1/4 mile numbers the G-tech reports speed the instant you hit the 1/4. At the drag strip speed it's an average of the last 60 feet. What this means is you will typically always get a mph number that is higher than you would at the drag strip. If you use it to calculate horsepower it only factors in weight and acceleration, it does not compensae for wind drag. What this means is the numbers will be lower than if you used a dyno an two cars with identical power the more aerodynamic one will get higher readings.
A buddy of mine has one an I have used it a number of times. You need a level strech of road to get the most accurate readings. Even though a stretch of road looks level you would be suprised how much it's not. Make runs both ways and use the average.
My buddy has had it on his windshield at the drag strip and consistently gets times that are within .1 seconds of the time reported by the strip. Here is whee there is another difference between the dragstrip and the G-tech. The G-tech starts timing the instant the car moves. At the strip timing does not start until you trip the lights so how you stage will impact the comparison results.
Overall I would say it's a great tool to quickly assess changes you make to your vehicle.
There is 2 different models. I have the G-tech Pro. The other one is the G-Tech Comp. The Comp is about 100 more than the pro i think. I think local places sell the Pro for about $140. If you want a Pro make me a reasonable offer.
I have Beltronic Accelerometer(Vector Fx-2) which might be as accurate as G-Tech one & I love mine cost me 176.00 with 40 rebate at Circuit City it rates Horsepower also but unlike the G-Tech but it will take into account drag coefficent on vehicle to give more accurate horsepower readings. it also requires u find a level strech road also and it has a scientific formula so u can figure out the drag coefficent of your vehicle included in the instructions(If u can't find it some were on the internet) you can also print out you best 0-60,1/4 mile times, & other info to your PC with(sold seperately pc cable) heres the link to there website I also Have the 9500 radar/laser dector which pretty reliable. http://www.beltronics.com/fx.html