While I've used a variety of heart rate monitors, my new favorite is the LifeSource XC300.  Not only can you monitor heart rate, you can also keep track of time in zone, calories burned and whether you've drifted out of your target heart rate zone.  It would be difficult to find a comparable heart rate monitor that offers so many advanced features at such a reasonable price ($109).  Programming the watch was a little difficult (you really DO have to read the instructions), but once I read through the booklet, I had it programmed in less than 10 minutes.  The transmitter is light and fit snugly around my chest with very little discomfort and my heart rate was displayed almost immediately.  It took me a little time to figure out how to maneuver through the different modes, but the large buttons made it easy to navigate even while running.

Perhaps my favorite thing about this heart rate monitor is the size of the display.   The numbers are big enough so that I could see it without moving my arm and I could keep track of my heart rate no matter what mode I was in.  The heart rate mode displays the time of day and heart rate.  The other modes (stopwatch, countdown timer and calories burned) also display current heart rate with visible arrows that blink if you're too far above or below your target heart rate zone.  You can opt for an audible alarm to sound when you're out of your zone, or you can turn the sound off and use the arrows.  Like other HRM's, the XC300 comes in only one size (may be a little large for delicate wrists), but it is much lighter than other HRM's I've used.  All in all, LifeSource's XC300 is easy to use and offers a wide variety of features at a very reasonable price.