I've had a cadence sensor in use almost since the 2nd month of cycling.

I was always in the 55-70 range and for the most part I was doing that in 50x14 gearing on a single speed setup. every other month my rpms would go up and my knees would hurt, but then I would rest for half a week. Still riding, but a slow pace. When I'd get back to upping the output I'd go faster and the knees wouldn't hurt. The process repeated several times.

My muscles grew and I was happy. I don't cycle in that low a range anymore, but my legs have also stopped getting bigger... but I am faster now with the higher cadence.

I think its all based on personal fitness level. The heart and lungs need to be conditioned before it really matters. In fact, If someone had told me this I really wouldn't have cared for the cadence monitor since I never really used it anyways except to say, "hey, look at that."

If I had tried to shoot for a cadence in the 85+ range early on... I'd be more fatigued than to make my workout of any efficient benefit. I remembering trying to spin faster (cuz thats what the pros do) and all I did on the single speed was excessively hurt my knees and when I converted back to gears... I just artificially raised it by dropping my gears to really low.

I like the poser factor of blowing passed people all decked out in pro kits with their high cadence and I'm just cruising along in the big gear. Feels good for the ego.