Well, I wouldn't trust it for the world to produce a slimming effect save for local dehydration. There is nothing special about the far infrared spectrum: both fatty and normal tissue will absorb parts of the spectrum and thus produce a nice, warm sensation. If the radiation flux is high enough so that fats begin to 'burn', you get the equivalent of a sputtering candle. Or of cooking meat---think an electric grill here :-). In other words: I'd take the 'fat burning' with a pinch of salt if I were you :-).
However, local heating does have some medical benefits. It soothes certain types of pain, and helps you relax. I remember that my parents had a cat whose favourite place was right in front of a normal glow bulb. I replaced it with an infrared lamp as it was friendlier to the eyes and more efficient at producing IR, and after that you couldn't get her to move with a crowbar :-).