Flax is one of the more beneficial supplements in my opinion and one that everyone should use if they just want a basic health supplement. Its main use is to get your essential fatty acids, mainly omega 3's and omega 6's. It is also shown to help reduce the risk of breast and other cancers along with cardiovascular diseases. I personally use the brand Liquid Flax Gold, which has a lot better taste than most brands. But they all take a little time to get used to. Best way to take it is just to mix it into your shakes, 1-2 tbsp a day.
You might also look into using Fish Oil to get your EFA's as it has a more complete coverage of fatty acids.
A really good basic faq on flax can be found here at BulkNutrition