How do you use your cadence monitor?

[Hammer]

I used mine last year during time trials... I'd shift between a couple of gears that would allow me to spin in the 90s range... I don't have a heart rate moniter... I tried to keep my level of effort up, so I wasn't just spinning an easy gear. Pushing a harder gear would've burned out my legs quicker and that's the best way that I could figure out how to moniter my effort rather than paying attention specifically to my mph. And I just went by perceived heart rate level anyway...

Our course is pancake flat, the only hard thing is turning around on a narrow road.

[makbike]

I use my cadence monitor a lot for it helps me ride more efficiently, IMO. I try to keep my cadence somewhere between 90 and 100 rpm on flat roads and somewhere in the 70's - 80's when climbing. That is not to say it does not drop below my desired ranges on flat or hills. I simply set my goals and try to maintain. I do find when I stay within my desired ranges I typically feel better good when I get off my bike.

[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)