USAir has a very liberal program which involves 2 System Wide Upgrades after only reaching Silver 25k.
CO/NW have great domestic upgrade program but with more and more people flying, even as a Gold I have a 60% rate of getting upgraded. I would sometimes prefer the electronic upgrades so that I would know a few days before that I am confirmed rather than mulling around the airport waiting to board last and then not having overhead space left.
DL, well, I had been accumulating upgrades over the years and I have gambled with the airline agreeing to pay a higher price for a chance to upgrade. Last year, I only got to upgrade as Gold twice. In case you don't understand this figure, imagine only 1 segment upgrade every 6 months!!!
However, all in all, I would prefer to spend my time in coach on AA before any of above mentioned airlines.
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"Fly me to the moon and let me earn alot of miles."