Calibretto: How much the detector detects depends primarily upon its mounting location. Most detectors are 360 degrees, so if you mount it in a location that it can sense radar energy 360, it can detect it 360.
And you don't want to necessarily turn down the sensitivity. Radar is line of sight, and often instant lock-on, so you want to detect the signal as soon as possible otherwise the cop will get an accurate read on your speed. Oftentimes you can detect the signals as the cop targets other cars, and those signals may be weak.
For example, cop sits at the bottom of a hill monitoring traffic coming down the hill and (likely) speeding due to the downgrade. The radar waves will, for the most part, travel parallel to the road, and thus up past the top and not down the other side (where you would be driving up the hill). Placing your detector high up in the car would allow aberrant and reflected waves (and thus weaker strength waves) to be dectected before cresting completely over the hill, thus allowing you time to slow down or not speed as you descend. If placed too low, your car will have already crested the hill before the detector detects the radar -- hence the desireability to place the detector as high as possible.
Thus, the first hint that a cop may be lurking around using radar is to "detect" the weaker signals from his radar as he targets other cars -- as once he targets you, you're toast. So you probably don't want to turn down the sensitivity of the detector, otherwise you may not sense the telltale signs that a cop is using radar up ahead.