I also have a heart rate monitor and I love it, it is a MIO or something like that, it is at home right now and I am at work, I think it is MIO. Anyways it has been great and lets me know where I am with my heart rate during an exercise. I have had it since Christmas and I can pretty much judge and gage where my heart rate is now without it, so if anything it helped me learn how to judge my heart rate and get it where I want it.
I have to say that the new workouts burn more calories for me than any other workout, but the Imax2 & 3 come close. But then again after the new workouts Imax 2 & 3 are as hard for me anymore. I was so amazed the last time I did Imax 3 and made it through.
In LIC Cathe says "you are going to be so physically fit, not that you aren't now, but you are going to better places" she wasn't kidding! OMG what a better place I have been taken to and I LOVE IT!!
The heart rate monitior would be a good investment if you are watching your calories intake vs calories burned, it can also help you judge your heart rate while working out and maybe even help push you harder at times. I know when I see a heart rate lower than I want I am jumping a little higher and pushing a little harder, it is a great motivator!