How do you use your cadence monitor?

[Kalidurga]

I use my cadence monitor a lot for it helps me ride more efficiently, IMO.

That's pretty much my reason, too, along with what Grog said about shifting gears based on my cadence. It's also a convenient tool for playing around with different training methods:

I used Sheldon Brown's gear calculator to create a chart I taped to my stem. Beginning with the knowledge that my average comfortably sustainable cadence is 90rpm, I can look at the cadence on my computer and know exactly what gear I'm in. Then, if I want to work a little harder and build some muscle, I can click up into a harder gear and try to maintain 90rpm. If I need a rest or want to work more aerobically, I can shift down to a lower gear and spin over 100rpm. Or, if I want to work on smoothing my pedal stroke, I can shift to that lower gear and continue spinning at 90rpm. Doing so without bouncing really forces you to focus on your stroke.

This article from Sheldon talks about both gearing and cadence, and gets a bit into what equus was explaining: Understanding your gears.

[HillSlugger]

I'm with Regina, I use cadence mostly on the trainer. However, outside it's often a reminder to me that I should be shifting to another rear (cadence greater than 99 or lower than 70)