Just check with your local laws to make sure it's not illegal to have one. Most places it isn't.

A good radar detector will be able to detect something like 80% of cops sitting on the side of the road. But just remember, if a cop pulls up behind you while you're speeding then hits the radar gun, obviously your detector isn't going to be able to pick that up fast enough for you to slow down.

If you drive on the highway often and prefer the fast lane, it's a good investment. But even with a radar detector, it's about 50/50.