Decision time - do I buy a heart rate monitor?

I've been thinking today that it is about time I invested in a heart rate monitor. I am not sure I know exactly what they do, or should I say how you use them but I feel that running at the moment, just looking at my watch is not really helping me improve. The questioon is, which one to buy. Also why is it that after you have submitted your Christmas list to santa that you think of a whole load of new things that you want? I have a Polar catalogue here that I am browsing as Mrs B has offered to take my "resting heart rate" in the next 20 mins and I am told that I need to be relaxed. Will run 9km today (a little later) which brings my weekly km total to around 34km.