I have the older version and as the other feller said, it's decently accurate if all the variables are correct. I have made several 1/4 mile passes at the strip with the G-Tech on board and it's close but not perfect.
For 1/4 numbers, you have to have the unit leveled right and make your runs on a flat surface {even better, make at least 2 or 3 runs in both direction and average}. Also, tire spin tends to make the results more suspect.
The MPH is always off. The reason is that the G-Tech measures mph at exactly the 1/4 mile mark. At the strip, they average mph over the last 100ft or so {can't remember for sure} so there is no way the two mph numbers are going to be the same. Most of the time, the G-tech is around 2-3 mph off.
ET can be pretty good if the unit is adjusted right {maybe a couple of tenths off}.
Horsepower can also be close if you input the correct weight of the vehicle {has to be exact, close won't cut it}. I made some runs with my Firebird when it was stock. It came up with around 300 HP, my Kia rio chimmed in at 80HP /forums/images/graemlins/bow.gif, the 85 Mustang 5.0 came in at 175 HP. Remember, these are rear wheel numbers. Pretty dern close! At least within 20 horses or so.
As I said, I have the older version that can be bought for around a 100 bucks or so. The new version should be better. Heck, it even has a shift light, built in tach and you no longer have to level the unit.