I always take mine when they come in off the track, with a trusty old stethoscope. I don't trust heart rate monitors because they aren't always accurate (and I know this because I have "checked" them with my stethescope and they're always out! If they're batteries are getting slow you'll get some funky readings too!).
Polar is a pretty good brand however. There is that old adage you get what you pay for
But seriously... a stethescope after trackwork is almost as good as a monitor during trackwork IMHO