Radar detectors can not give you 100% protection. It's important to understand their capabilities and limitations.
When police officers are using radar up ahead, radar detectors can provide plenty of advanced warning to let you know before the officer is able to acquire your speed.
However, if the officer is using instant on (turning his radar gun on and off), you may not get any advanced warning. Using a rabbit and a sensitive detector are key for detecting instant on. See this video for more info.
Additionally if they're using some other technology like laser, vascar, visual estimation, or air enforcement, your radar detector won't reliably protect you from those. That's why it's important to have multiple layers of protection. I recommend running 1) a good radar detector, 2) a good laser jammer, and 3) Waze.
Radar is actively used nationwide though and so when officers are using it in your area, your radar detector will be an invaluable tool. I personally get more saves from my radar detector than any other countermeasure.