I have a Garmin with a heart rate monitor. I don't use it a huge deal other than for tempo runs (i run these at a specified heart rate and don't focus on my pace), and in races to check I am not overdoing it. Other than that I tend to focus on my pace for long runs, easy runs and intervals, and just scan the heart rate data out of interest. Certainly a useful training tool, and many train by heart rate rather than pace successfully