there are many different factors that come into play with a radar detector.
first, you get what you pay for... same addage used anywhere else with cars - it stands true here too.
second, its not a free pass to speed. Yes they work, quite well actually, but you cant take the human out of the equation.
if they dont have their radar on, you arent going to detect them. If they flip instant on (still giving you time to slow down, cops cant use pop radar as an accurate measure) you wont see them till it comes on.
Then there are those people that hate false alarms soo much, that they ignore the real ones. I had a passport 8500... best money I ever spent on my car. Not once during the 2 years I had it did I get a ticket (someone stole it), and I had TONS of times where I would have been pulled without it. Its funny how many cops you find with a radar detector that you would never have known existed without.
Buy a radar detector, but here is my advice:
dont go cheap. My recommendations are passport 8500 x50/9500, valentine 1, or Bel (dont know their higher model #s). If you go cheap, they wont have as far of a detection zone as the good detectors... valentine 1/passport 8500 have up to a mile range depending on conditions. The cheap ones? as low as 1500 feet. Also the cheaper units usually wont have pop radar ability, laser detection, or passive technology.
USE YOUR EYES. There is no replacement for being careful and still paying attention. A cop can still pull you over for tons of reasons without using a radar. A detector is only going to protect you from SPEEDING violations.
Drive safe. Just because you have a radar detector, doesnt mean youre going to get away with doing 120 in a 25mph zone. Remember, there are tons of varying aspects that you have to account for with radar detector. If that cop is sitting a mile ahead just blasting radar, yeah youll see him (with a good unit). However if a cop is on an isolated road using instant on, well youre probably screwed if youre going 120 and cant slow down in time.
Laser = instant ticket. Unlike radar, laser is a very focused targeting system. If you see laser flash on the detector, 95% of the time hes targeted you directly. All the radar detector is letting you know is that you should pull over if youre speeding cause that cop already has you.