One of the biggest benefits as far as I am concerned in the guaranteed seat. I can't count the number of times I have used that perk to get on a full plane in C-class. In fact I'm doing it tomorrow from NRT to FRA.

It's also pretty good the way things suddenly become possible. My travel agent had got me a window seat. I called LH to change it to an aisle. No luck. I then checked to see if my FF number was in the booking file...it wasn't. When I gave it to the agent...suddenly an aisle seat materialised.