There has been much debate about the usefulness of fish oil over the last couple of years. Some research shows benefit and other research does not. Unfortunately, when one looks closely at the studies often the very fish oil is to blame. To often poor quality fish oil is used and in amounts that are of no real benefit. I remember discussing with a pharmacist about the fish oil they were selling. Her face dropped when I suggested that my patients would have to take 20 capsules a day to get enough DHA and EPA.
In regards to the benefit of fish oil and mental function, there is an interesting research study done by Tuft's University. In the study they found that grouping people on the basis of EPA and DHA consumption those with the lowest intake (lowest 25 % of participants) had the greatest cognitive decline over a two year period.
One of my biggest concerns is the quality of the oil as well. Almost all fish oil is tested for heavy metals, but not necessarily PCB's. Also, the cheaper the product the poorer the quality of the oil. If it is not converted back into a triglyceride when it is refined there is no real benefit in taking the product no matter what the DHA and EPA levels are.