Measuring and using heart rate as a training and riding tool is useful, but only on a personal basis. Such measurements are nearly useless as a comparative with others. Plus, heart rate can and does change due to a variety of circumstances. Because of that many people use "perceived exertion" as a more appropriate and useful tool.

In my personal case I'm buying a heart rate monitor to use because I've discovered by borrowing one that it is a useful way of pacing myself over longer rides. By correlating heart rate and cadence I can ride maximum pace while not becoming tired too soon. That way, even though my mind is saying either "your going too fast", or "you need to speed it up" I have objective numbers to make the decision on.

Interestingly, over the past couple years I've been able to raise my "cruise" heart rate by 20bpm while my muscles feel no different and my average speed has increased by nearly 20%.