Well having recently being diagnosed with Hashimotos disease, it has been brought to my attention that I should maybe start to train with a HRM. I have always a avoided HRMs as I was good at going by feel and never saw the point in needlessly introducing science into my training. I have now come round to the idea and have decided to take the plunge and buy a HRM, which is probably going to be for the best considering my tendency to over do it and cause injury.
I'm glad I've started doing a bit of reading around the subject however; as I was always thought that you go out, get your max heart rate and Bobs your uncle.
Further reading however has suggested that lactate threshold is a more accurate (not to mention safer) way of doing things, and the route I intend to take.