Previous studies have shown that it can help to keep fat off, but doesn't help to lose it, though one study showed an increase of a half pound a month extra lost. (edited - 0.8 lb per month which is almost a pound, not a half pound lol)
However, recent studies have shown potentially serious side effects. Every one I know that was taking it has stopped.
According to an article in the NY Times
Researchers have found that it can increase blood levels of C-reactive protein, lipoprotein and leptin — all of which can heighten the risk of heart disease. There is also some evidence that taking C.L.A. daily increases insulin resistance, a sign of impending Type 2 diabetes.