I use the wired Cateye Strada Cadence computer on my road bike. The only reason I got it was so I could tell how I was doing on the trainer (speed and distance wise), since a normal bike computer won't work connected to the front wheel. After I got it, I started keeping track of my cadence because of all the reading that I have done on the subject, especially here on BF. Since I ride my road bike and the hybrid and I don't always ride them in the same manner each time, I started to pay less and less attention to the cadence and now seldom use that mode.

On the road bike, I pretty much have the 80-100 rpm's down to where I know if I'm pedaling too slow without using the computer. However, there are times when I ride and I am not doing more than 10 mph (neighborhood rides with the wife or rides with the grandkids) so trying to maintain a cadence of 80-100 is a waste of time.

Edit: My wife has the Garmin Cadence sensor on her tadpole trike to go with her Forerunner 405CX. It is more noticeable than on a DF bike because it is mounted at the crank, which is out in front of everything else. She uses it for the cadence only.