RT2, I found my HRM really useful when I started out, and when I wanted to up my pace. Your MAX heartrate should be 220 minus your age, e.g. 220-21 = 199. If you get to within a few beats and stay there for too long you will probably collapse in a heap. So the idea of using a HRM is to help pace yourself to keep as low as possible.
I found that once I'd established a pace i didn't need the monitor. At the moment I am getting over a virus and my attempts to do a 4.30 at Lochaber in a couple of weeks have been severly dented.
Depending on how I do (hopefully finish!), I will set my pace for Berlin. If I do go for a faster pace (which I intend too), I will use my HRM to pace my Fartlek runs. Sprint until HR goes high, walk until it drops, jog keeping it steady. This , for me, is it's 'killer app' as they say.