It's definitely helpful, I haven't been sick for over a year and I have a much stronger sense of well being (no depression). My blood serum levels were optimal as well (in other words, at the top end of the range because quite frankly, our society is vitamin D deficient).
I take fermented cod liver oil from Green Pastures and I highly recommend it. Actually, it's the only CLO I would take. Between 5000 UI (1 teaspoon) and 10000 UI a day.