Here is what I've heard about flax seed, although I have yet to use it myself (I've got some, but haven't used it, yet):
If you have not used flax seed before, you should start out slowly. It is a wonderful cleansing food that detoxifies your system and will royally "clean your clock" if you aren't accustomed to it.
The lady who I talked to about it told me that a lot of people who have never used flax seed think that more is better and get very sick from using too much in the beginning. She said that you should start out small, then work your way up to larger quantities. I've got some ground flax seed in my fridge right now, and I think I will start using a half teaspoon at a time since I have tender intestines, anyway. If a half teaspoon doesn't bother me, I'll work up to a teaspoon and so forth.
This is just a word of caution from someone who's been given a word of caution from someone else.