heart rate monitors and exercise calories

I know the topic has been discussed and I did do a search but could not find an exact answer. For those who use a heart rate monitor, how accurate has it been for knowing how many calories you are burning from any type of exercise? Meaning, if you ate your calories burned from exercise back, have you met your goals? I have been using my Polar FT4 for the past few days and enjoy the fact that I can see how many calories I have spent in a session. I am just really wondering how reliable the counts are as far as what is ACTUALLY the case. Thanks!