Although I'm finding myself quite sceptical about it I've been following the Maffetone approach last winter during the aerobic base building phase of my training.
So I use the HRM to make sure that I'm not going over the point where Maffetone says you start becoming anaerobic which is 180 minus your age. in my case 144 Bpm.
This is very interesting, it certainly puts a different complexion on training pace, if I used this system i would need to keep below 120bpm to stay aerobic.
Not sure my training partners would let me run that slow

It might explain why I can't get a decent sequence of training sessions going though. Might try this out next winter.