I think it really depends on what brand it is, what it's made of, etc. I've tried them (I don't have the bottle with me, so I can't tell you what's in mine), and I have a physician-friend who has tried them and said that he would use them occassionally, but he wouldn't make a regular habit of using them, so I follow his advice on that.
I only use them once in a while when I eat something high in carbs. I don't believe they help in terms of losing weight, but it seems to help at least with maintenance.