Hi Newbie!
You are probably like me and track your calories on a daily basis or as much as possible. For that reason, I wear a heart rate monitor for nearly every exercise I perform. Hot yoga is an exception to this because I feel it causes distraction and discomfort. I decided to wear it for one class and get my total calories burned and just go by that number every time.
As far as the heart rate monitor being accurate – from what I know, they are most accurate with traditional exercises that your body and muscles are used to and exerting yourself to unfamiliar or extremely strenuous exercises can throw the calorie count off. I still, however, feel better knowing that I have a set calorie count in my head of what I have burned after a class. It might be a little more or a little less but overall I feel satisfied.
I use the PolarFT7 and love it. It monitors the duration of the exercise, the calories burned, heart rate, and how long you were in fat burn or fitness improving mode.