I have a rear wheel computer with cadence. It works great, especially on the trainer. The down side is you have to run cord down your frame to the back wheel. Small price to pay for knowing your stats.

I couldn't live without my computer. If you are doing specific training (especially if you are racing), you need to know your stats, including cadence (very helpful when doing speed and power intervals). Some people like to go by "feel", but 90rpm can feel different depending on how recovered/tired your body is (which is very similar to heart rate....175 bpm can feel comfortable or it can feel like death depending on how recovered I'm feeling).