I couldn't be more positive that they do work. Being a Pharmacist, I investigate claims thoroughly before either recommending or refusing supplements, and must say, most do what they claim; at least to a degree. When considering fat burners, for example, many of the ingredients are proven to increase your metabolic; hence you burn more calories through sweating ( and the name thermogenic for those of you that don't know).
Secondly, some ingredients may prevent the storage of fat (such as chitosan) or inhibit mechanisms that blunt fat burning (such as yohimbine). What I am pissed about, however, is the fact that these big supplement companies do not even give you a clue as to what you should be eating to see these benefits, and sadly, many persons do not.
If you should sip Gatorade throughout your gym session, do you know you are inhibiting your fat burning? Most likely no, or you wouldn't be. They should advise people these things and they would truly have products that no one can say is bogus.