I remain skeptical. I checked out the page where the company explains the mechanisms used:
BODYBUGG :: THE SCIENCE BEHIND BODYBUGG
In essence it tries to estimate your caloric expenditure and claims that it's "the most accurate estimation [...] outside of a clinical setting". This could mean anything from "very accurate" to "not very accurate". I would say that since how much you sweat, how much heat your body gives off, how much you move your arms etc. can as a matter of principle not give you accurate numbers. Especially considering the accelerometer, if for example you're doing squats, it would have no idea of how much weight you are lifting.
So: It's most likely not very useful, if you're looking for something accurate. AFAIK there really isn't an meaningful way to do this outside of clinical settings.