Its just vitamin A. Its very low in omega 3's, so if you are wanting this instead of just a source of vitamin A, use salmon oil (most fish oil supps are salmon).
Cod liver has benefits for your joints, I have personally seen it's work:
"Cardiff-based researchers say taking the supplement could delay or even reverse the destruction of joint cartilage and inflammatory pain associated with arthritic disease. "
As an EFA, it doesn't have the degree of EFAs that flax would have, so I would stick w/ flax or UDOs.
It's good to take, I wouldn't limit myself to just cod liver oil though.....it's a good source of Vitamin A, D, and has decent amounts of EPA, ALA, and DHA