I have changed FFP, not because I was unsatisfied with AF, but because I have moved to DXB. So naturally I am flying EK.
But honestly it opened my eyes.
Starting from the airport it is a whole new experience: the lounge is amazing (all kind of food and drinks, usually welcoming staff) for instance.
In the plane as a gold member you get greeted personally, served food first and get asked if you need something. Once I asked if I could get some of the delicious business class chocolate (I was in Y) and the purser came back with a bag full of Godiva chocolate! Yummi
then the upgrades: i get upgraded a lot more then on AF. On my flights since October (about 10-12 segments) I have been upgraded 6 or 7 times.
redemption on skywards: there are nearly no taxes to pay, so the flight is for free indeed. You can also buy upgrades with miles regardless of your booking class.

All this does not make me wish to fly again with AF, apart from their quite good long haul premium economy hard product (but again, the service is sub par usually...).