After reading a thread from a guy who claimed he had a slow metabolism, and performing some searches, I stumbled upon this:

http://www.healthetools.us/bodygem/index.html

This device, called the BodyGem, tells you your RMR (resting metabolic rate = the number of calories you burn by doing nothing daily), by just breathing into it.

I'm shocked because I had never heard of such a product.

Does anyone have any experiences with similar apparatus ?

From Their Web page, this looks like an interesting device - seems to calculate your caloric burn based on O2 and CO2 in your exhalate. However, $1999 for the unit and 21 masks? That's ~$100 per use. No thanks.

The thing is, this is meant for coaches and personal trainers. Hence the need of many masks. If you were to use it for your own, naturally you'd reuse one.

I think obviously the point is not buying it but using it at some fitness center or gym that has it.