I use Valvoline. At 50k miles, I dropped the pan, replaced the filter, and refilled the fluid. Mine took almost 8 quarts. About 6 months later I had my son, who works part time at Valvoline do the flush during one of their specials. The fluid was still pretty nasty that came out of it, so I'll be dropping the pan to replace the filter again.
I ran the "flush or not" question past a mechanic at one of the trusted local transmission shops. His opinion was that a flush was a good idea, as long as the fllter is also replaced. When they do a flush, they also drop the pan, replace the filter, then do the flush, and do it cheaper than any of the quick lube shops. He said that unless the tranmission is torn down to drain the torque converter, even a flush doesn't replace it all. As to "killing" a transmission, he felt that if that occurs, the transmission was doomed to fail anyway, and would only hasten it by about 2 to 3,000 miles.
Black 2000 SSEi, Silverstar Headlights, Angel Eyes fogs, 3rd brake light overlay, hi-flo cat, drilled 180 degree thermostat, Intense FWI, PCM, shift kit, and 3.4 Pulley. Solid front mount.