Never had one in 20 years of driving. Only got 2 speeding tickets in those 20 years (well, at least until I just wrote this, surely earning me a "jinks"
) I guess I just don't speed enough.
Then again, for the most part, I drive familiar roads ( from VA to MA) and have done a good job of remembering where the speed traps are. Plus, I find myself a more alert highway driver by constantly scanning ahead with my eyes to spot them. Doesn't work all the time, but as mentioned above, it hasn't worked too poorly either.
If a $300-600 peice of equipment buys you the piece of mind to drive the way you want, then by all means get one.