My ex-girlfriend had one and never used it. I tried it a few times and it was kind of interesting to get overall work out data after everything is said and done, but kind of unnecessary and a little too time consuming to be worth while. I monitor my heart every time I work out by counting my pulse for 15 seconds and multiplying that number by 4. I normally just use my CD player clock for timing and my neck vain for my pulse. I find this technique to give an more accurate and instantaneous reading than most monitors.
Now I only monitor my heart before a workout for my "resting rate," which you want to be low. Then during endurance cardio work. I like to run up stairs as fast as I can, then walk down them and wait at the bottom to recover. The better shape you are in, the faster the recovery. Heart rate information is key to this exercise.
Oh yeah, if you take any kind of work out upper (red bull, hydroxy cut...), always monitor your heart rate and never let it get too high. When on uppers, I always try to get it under 170 BPM.