They're very accurate. I have a Planet Bike Protege 9, I got it because I was reading travellogues on CrazyGuyOnABike.com and liked the idea of having a thermometer on the computer. Also it's waterproof, my last computer flaked in the rain. I get a kick out of watching it go over 100*F on the ride home.

When you install it, put the magnet and sensor in a spot close to the hub. If you put it near the edge of the wheel, not only does the magnet unbalance the wheel more, but it flies by the sensor so fast that the sensor starts losing counts; until I figured this out, my comp started jumping all over hell when I got above about 20 MPH.