In reply to Paul Robertson:
I don't think they're especially accurate and I don't really pay much attention to the calories. If I load my Forerunner data into Garmin Connect, Fetch Everyone, and Strava, they all come out with different calorie expenditure for the same run.
However, Strava does seem to work out the difference between runs and it kind of 'looks' right. Eg.
12.6km with 343m of ascent, 1hr 07 - 1,074 calories
13.0km with 834m of ascent, 1hr 46 - 1,606 calories
I think there are so many factors involved in accurately calculating calories including your age, weight and heart rate, and how cold it is, that this can be nothing more than an educated guess.
I just know that I run a lot and eat a lot and can hardly keep weight on so I'm easily burning enough - right another piece of cake please