Coenzyme Q10 supplements may be beneficial, but should not be relied upon.

Coenzyme Q10 is a vitamin-like supplement which plays a vital role in producing energy in cells. It is also an antioxidant. Concentrations of coenzyme Q10 do tend to fall with age, and they are also low in Parkinson's disease, diabetes and cancer. That is why the value of coenzyme Q10 as a supplement is being promoted widely.

Doctors at the Mayo Clinic point out that there is not much scientific evidence on whether coenzyme Q10 can prevent or treat disease. Most studies have been small and have not had a group on placebo. However, some studies have shown evidence of benefit in Parkinson's disease, migraine and high blood pressure - but more work is still needed. So, if you do want to try coenzyme Q10, discuss this with your doctor and regard the supplement as supportive, at best. It shouldn't be taken alone to treat a medical condition.

Source
Mayo Clinic Health Letter November 2006