Garmin Calorie Counter

I was wondering how accurate the calorie counter is with a garmin watch. You have to enter your weight and the heart rate monitor helps measure the intensity of your run. Does anyone have any insight to how accurate it is?? For example, after my 10 mile run on Saturday it told me that I burned 1,398 calories.

I don't use it at all. I just estimate that I burn 80kcal/mile in addition to an estimated BMR of 2400kcal/day. That said, the Garmin estimate is usually somewhat accurate for me if I add back the 120/hr (estimated waking BMR) to the 80/mile. Meaning the Garmin will show that a 15 mile run that takes 2 hours is somewhere around 1440 (15*80+2*120).