According to this article, coffee made with green (unroasted) beans speeds up fat-burning in the body because it contains high levels of chlorogenic acid, much of which is destroyed by heat during the roasting process:
Since chlorogenic acid is an antioxidant and reduces glucose absorption, it would seem to be a beneficial substance. However, according to this study, it can also increase homocysteine levels in the blood.
Also, since caloric restriction is associated with a lowered metabolism, I wonder if raising metabolism (with chlorogenic acid or with green tea, say) might have a negative effect on longevity. Or would a decrease in body fat make up for that?