I thought of buying a HRM until recently when I realized how gosh darn expensive they are and how easily they are screwed up by the slightest electronic insult.
I've decided on the 6 second count of the thumping jugular instead. I do it while running to get an accurate idea, and you can usually tell if it's 150, 160, or somewhere in between (Say 155). That's accurate enough for me, and it's free.