My accord has one. It does two things. It remaps the throttle so it's less sensitive, basically it takes pushing the accelerator more to get the same response as if it was off and it also makes the AC compressor cycle on less often. Some people have noticed it makes the transmission shift earlier but I have not noticed that.
I used it on my drive across Canada and I can honestly say I noticed no difference. I got 6.5L /100km with it on and off. That's my experience with it in a V6 accord anyway.