I have used the Gwoof since November and I do find certain aspects of it quite useful. However, I find its numbers somewhat suspect, as it really doesn't seem to be in line with actual effort. For instance, you can half kill yourself with a two hour weight training workout and get a grand total of 350 calories burned. You can go for an easy run or even a walk and get a huge number. For dancing, it says that I burn up to 750 calories an hour! It thinks Insanity is only somewhat vigorous, while kickboxing is hugely vigorous. It doesn't use heart rate as an indicator. Basically, it really rates certain types of movements as high calorie burners and others as very low, even though that is not the subjective experience and really doesn't match what other calorie-burn calculators would tell you. It rates steady state motion as burning more calories than interval-type motion. It is good for tracking sleep, but it will make you paranoid about not getting enough of it.
I also have a Polar, and find it very sensitive to your settings.