I have the Vector FX-2, but haven't used it much yet. There aren't too many places where I can do a proper launch for a 0-60 measurement, and even fewer where I can keep going to 90mph for a 1/4 mile.
They claim to be fairly accurate, but I wouldn't put a lot of stock in the numbers until you can get to a track at least once to make sure it's set up correctly (accelerometer times match track times). However, even if the numbers are off a little bit, it will still tell you if you are improving.
Even without doing a timed run, it's kind of neat to see what kind of G-forces you are pulling during acceleration or going around curves. I just wish they had a cordless version so the cord wasn't hanging in front of the center console; I may have to explore a hardwired installation.