Get the best one you can afford, because they DO work.
I wouldn't say they're perfect, but the one I have (hardly the best one either), has saved me from a few tickets so far. Whichever one you get though, there WILL be some false alarms, but after a while of driving around with it, you begin to memorize the pattern of false alarms on your daily route, so if something beeps out of the ordinary, bingo! it's probably a cop