I use one occasionally but not on every ride.

Two ways of looking at them--

You can get the all singing dancing version the works every thing out for you and rings a bell when you are not working hard enough

Or you can go basic and just have one to tell you what your HR is currently and the max you reached on a ride.

I have had both and all I want the monitor for now is my current HR. That is only so I can judge if I am working hard enough or working too hard.