Badger6

I'll offer my opinion, which is nothing even remotely close to consensus...a waste of money and potentially more downside than up. As some others have said, feel is more important. Two primary reasons:

(1) Grade. The optimal cadence to climb is different than descending is different than riding a flat (+/- 1%). I'm not a doctor, a kinesiologist, or a pro/coach, so I don't know why, but I do ride with a power meter and I can tell you that power output is the key, and you use different gear ratios and cadences for each of those three gradients (climb, flat, descent) to achieve the target power.
(2) Feel...like perceived exertion. Some humans can spin high cadence all day, some can't. How do you know what your optimal cadence is if you don't know your FTP, and then can use that to determine the optimal power, which is a function of cadence. Ride what feels right for each situation, and then experiment with it. Until I started riding with a power meter, I used a cadence sensor...in my experience, over many years and several hundred thousand kms, when I paid attention to my cadence, I was always chasing the number I thought I was supposed to be at, and miserable; when I wasn't being bothered, I was riding where I felt comfortable and I was faster and could go longer.

That's a really long way to say, I wouldn't recommend a cadence sensor. However, if "feeling" is not the route you want to go, I would recommend a power meter, for anyone who is interested in riding for anything more than family rides and putzing about the village. It's far more useful, and with a basic understanding of what it's measuring, and understanding the feedback it is offering, it will offer some significant and dramatic improvements and positive changes to your cycling experience, whether you have amateur racing dreams or just want to conquer epic rides.

My 2 cents...