has DL been tremendously successful with these fares...selling seats that would have otherwise gone empty? my gut says no, but i have no data to back it up and i'm not sure such data would be publicly available.
seems that UA and AA (and now AS with "premium economy coming") are content to copy anything DL does, regardless of whether DL's changes are successful or not. DL is simply seen as the sexy girl on the block, which yes they're the best, but only the best of the worst.