"For all I know" is often used in situations where the statement that you are qualifying with this phrase runs completely contrary to what was said right before that, or is a wild guess that would change everything compared to what was said before.
I don't know why his opinion of these products is so favorable. He could be an investor in the company that produces them, for all I know.
StoneyB has given a rather comprehensive answer on "for all I know" on ELU.
"As far as I know" is not used alongside guesses. It means that you are offering information that is based on what you actually know, but you don't know a lot, or you could be mistaken.