There is evidence (though it's not conclusive) to show that CLA can help you keep the fat off once you've lost it, so it's definitely something worth considering. However, it has not been shown to help you lose weight. It only helps keep it off after you've lost it.
According to a study at the University of Wisconsin:
Dr. Michael Pariza, who conducted research on CLA with the University of Wisconsin-Madison, reported in August 2000 to the American Chemical Society that "It doesn't make a big fat cell get little. What it rather does is keep a little fat cell from getting big."
Pariza's research did not find weight loss in his group of 71 overweight people, but what he did find was that when the dieters stopped dieting, and gained back weight, those taking CLA "were more likely to gain muscle and not fat.'' In a separate study conducted at Purdue University in Indiana, CLA was found to improve insulin levels in about two-thirds of diabetic patients, and moderately reduced the blood glucose level and triglyceride levels.
The effects of long term usage are unknown. Side effects include mild gastrointestinal complaints such as nausea.