I have used a HR monitor for the last year.
I achieved a 3 stone weight loss (from 18.5stone to 15.5 stone) in around 3 months by exercising on CV machines for around 1 hour 4 times a week at a heart rate of 80%.
I did also generally eat very healthily - I actually follow weight watcher points and make sure I don't eat more than 40 points a day. This proved difficult in December since I was out a lot - and then the gym closed for two weeks and it was Christmas where I did binge.
Am back at the gym again and back on the healthy diet.
I found that if I eat what i want and exercise, I maintain my weight, If i take care of what I eat, I lose weight.