The nytimes.com has a great article on exercise, heart rates, and heart rate monitors here: The Flutter Over Heart Rate – New York Times
I am glad I read it. I have always been embarrassed about my heart rate when I run, because it gets really high. The formula of 220 – my age gives me too low a rate. In marathons I like to get it around 180, and in 5-10K I can crank it up to 200! But I always thought people would see it and say: whoa, that’s crazy! Or you are out of shape. So I kept it a secret! But when I read the article, I thought: ok, there is a wide range of possible rates, then.
I have a HRM from Polar, it’s basic (like the one above), but that’s enough to help keep me focused. If I feel tired, I will tend to slow down, and sometimes I don’t want to. The HRM helps there. But in a race, it does the opposite: it helps me keep my pace. It’s too easy to go too fast in a marathon or a half. The HRM monitor keeps me in control.