I disagree with the people who say don't bother. For a couple hundred bucks you can see around corners. I haven't been able to activate my personal X-Ray vision yet so I can see through overpasses and rock walls, so I'm forced to rely on radar detectors. Mine has saved me at least a dozen times in the past 10 years. Not all cops constantly use laser or instant on - I can tell, through my detector, that a good number of them simply turn on the radar and kick back and wait for someone not paying attention to roll through, which happens all the time. Furthermore, I pick up signals when the guy in front of me gets hit. That is often enough to drop a couple MPH off and avoid a ticket.
I have had 3 times in the past 5 or 6 years that my detector went off, I slowed down to 5 or less over, some guy went rolling past me, and got pulled over as we went around the corner. It's not a false sense of security if you use a radar detector as part of an overall strategy. If you think you're just going to go 90 and wait for the detector to go off, you're going to get some tickets.
Anyway, on detectors. I have a Bel 916, which is an older model - 12 years old - and it still works flawlessly. I will probably replace it with another Bel detector when it no longer works since I've had such good luck with them. I have also heard good things about the Whistler remote setup, which is nice because you can mount the display anywhere. I'm talking about detectors that only run a few hundred bucks. If you want to go all out you want to probably go with the Valentine or the Escort.
PS: Knock on wood, I have never had a speeding ticket. I have used a radar detector my entire driving life. Coincidence? Maybe.