I've been in FFP's for around the same length of time as the OP and my redemption frequency is very similar. I've rather given up trying to attribute a monetary value to it all because direct comparisons with revenue fares is a very inexact science but I would certainly have less money in my bank account now if I had paid for all the trips I've taken.

I have an uneasy feeling that the good aspects of these programmes will continue to be eroded while increased charges and ever decreasing availability will make them less attractive to many of us.