Try and get a 2nd hand one. Its only accurate if used on a totally flat road, with no bumps as it works by sensing accelerative and braking g-forces. Bumps and changes in incline WILL throw it out. It also takes a fair bit of calibrating if you want good results.
Having said that, I tried mine at York Dragway a couple of times and it was accurate within 0.1 seconds over a 1/4 mile.
So get one if you have a decent amount of flat roads around you. If not, itll be a waste of money.