i think everybody here has great points. But i do agree also that everyone has different criteria. As a FF who travels mainly long haul internationally in Business my criteria lean more towards inflight service / courtesy / comfort / aircraft quality. I also am more concerned with ground services such as lounges and the ability for my airline to take care of me in th event of weather or other such delays. Upgrading is nice (definitely more so for continental US travel) but there are many other issues which are equally important. i guess what i'm saying is that if youre happy with the overall service of your chosen airline then that should be the defining criteria.

sorry for the rant just my 2 cents