My 3 cents:
AA - unreliable, inconsistent service, formerly excellent FFP now reduced to rubble, formerly excellent alliance now fractured thanks to BA et al.
UA - outstanding alliance partners at least in terms of award availability, middling FFP, seem intent on actually improving on-board product, but think elites are over-entitled (though it seems USAirways is also headed down this road)
DL - profitable, reliable, mostly worthless FFP, decent product, weakest alliance
I know next to nothing about B6, WN, and the others.