I have a Quarq PM and a Garmin computer. When I’m doing intervals, I have a display that shows 3 second power, avg power for that interval, lap (interval) time. Sometimes I will add a 4th field for cadence or HR. I can set up the Garmin to guide me through the workout, but that can be a PITA so I usually just write it on a piece of tape and stick that on my stem.

It’s a far better value than a new set of wheels if you use it to follow a training plan. It can also be an easy way to get on zwift or TrainerRoad if you have a basic trainer. But only if you actually use it for training. If not, it’s just an expensive gadget.