I have a radar detector in my subaru currently. The biggest mistake I see people make with radar detectors is they put it in their car and think "woooooooo i can do 100mph and not have to worry". Problem is, most intelligent cops dont just sit there with their radar gun on constantly. They pic a target and beam at it. So if you're on the road doing 100mph, and there is no car in front of you, guess what, you're not gonna pick it up cause he's not using it on anything, until he sees you go by. Which inevitably leads to a nice hefty ticket.
You still need to use your brain about it. Mine has saved me numerous times, but not from doing 100mph in a 60. more like 75 in a 60. They're a great addition to the car and help up your chances of speeding successfully, but they're far from fullproof.