I have two: a Quarq crank on my main road bike, and a Stages left arm on my back-up trainer bike). I guess 3 if you count the trainer itself. If you want to follow a power-based training program, it's pretty much a necessity. You need to test yourself and set wattage ranges for different training zones. You can really geek out on it. It will also keep you honest on your recovery days.
It can also be very useful if all you want to do is to track calories. It will show you pretty precisely how much work you put into the bike on your ride, whether it's up and down hills, or into or with the wind, or solo or with a group. We tend to overestimate how much work we actually do, at least I did. "I rode for 2 hours! So I can eat whatever I want now!" But a PM will not lie: A watt is a watt and a kilojoule is a kilojoule.
Last edited by caloso; 04-02-21 at 11:00 AM.