A GPS is definitely not necessary, but I am so happy that I have mine. I started with a Garmin forerunner 305. It uses a heart rate monitor and I bought a speed/cadence sensor for $35 to sync with it.

I used the heart rate reading and cadence reading to select the appropriate gearing. I easily downloaded data from the ride. I also used the Cateye Strada wireless and was happy with it. The Garmin was never impacted by my Magicshine, but the Cateye would go crazy whenever I turned my headlight on.

I upgraded the Garmin to an Edge 800. It was worth every penny to me. It was compatible with my 305 heart rate strap and the speed cadence sensor. I really like the color touch screen and the maps I downloaded for free. I can see the data much more clearly on it than I could on the Cateye or Forerunner.

I have been monitoring my cadence for many years. I know when mine is low or high, but I can not guess it within 10 rpm.

I like to do intervals where I push my heart rate into zone 5. I shift to a higher gear and watch my cadence and heart rate. I can't do that with the Cateye.