I bought two cheap one's and am on my third one now. The first one (a Uniden) would go off at any RPM level above 2500. It was totally useless to me so I took it back. I realize that it was because I have an old car with a point ignition system, but still, they could have done better.

The second one was a Bell, it was very quiet. In fact when I drove by one of those signs that tell you how fast you are going it wouldn't even go off. I drove around for a month expecting I was protected just to find out it was broken. I took it back and got another Bell. It seems to work fine and has saved me a few time (except for instant on radar, damn them).

Just make sure it works before hitting triple digit speeds.