I've got a Garmin HRM. It talks to my Edge bike computer. This means I can look down while I'm riding and see what my heart is doing, and it means I can politely ask the computer to yell at me when it doesn't like my heart rate. Sometimes I have it nag me when I go anaerobic or redline, which helps me keep a nice, relaxed pace, especially for a recovery ride. Other times I'll have it complain when I don't work hard enough. I don't use the nag feature often, but it's pretty useful when I do.
On the other hand, REI's used section has lots of Sigma and Polar heart monitors, and some Garmin watch ones, for very good prices. They don't do quite as much, but they'll tell you your heart rate.
So I guess the real question is what you want it to do for you, and, also, how much you'd like to spend?