So, my figures are not exact, but over the the last 9 years I have probably used our companion pass on about 400 flights or so. Using an average ticket price of $150.00, cost savings with companion pass is roughly $60,000 over that period of time. (I also currently have just under $5k worth of flights reserved for the rest of the year on CP flights).
In my opinion, the companion pass is the best deal in domestic US (and now Caribbean travel) if you are a DINK who likes to disappear on weekends as much as possible.
Of course, if you have a family and can only travel for leisure a few times a year, you would likely find a better value in other travel currencies (SPG, Amex or UR).