Yup, I've got one Dave, and I like it.
It's accurate and reliable alright.
The speed it indicates is determined by the wheel circumference you enter. I have set it close to the real gps speed.
Very handy if you're following a roadbook. You can reset the distance since your last turn with one push of a button and you can adjust the total distance driven.
There's a bunch of different odometers, you can see oil temperature and battery voltage, distance since your last fill-up, economy on you previous tank.
The only less accurate thing about it is the outside temperature.
If you get one, make sure you take the remote control too, because you'll end up using only half the features if you don't. I have wired an old radio remote off een LT, but TT sells their own.