I have a cadence sensor as part of my cateye computer and have found it useful. I think it helped me to learn to spin the pedals more rather than mashing them in a bigger gear.
I also use a Garmin 310xt on my bike but haven't bothered with the cadence sensor, if the cateye one dies I'll get one then.
Edited: 04/01/2013 at 14:45