DL has 16 md-90s. Their plan was to phase them out by 2006 for a number of reasons including dispatch reliabilty, parts availability, and fleet simplification. I'm not sure what the current status of the phase out isnow, but I know they'll still be with us after 2006.
They really are not a bad airplane but they weren't produced in numbers sufficient to become a great airplane. Support is lacking because Boeing doesn't put the same effort into supporting a discontinued MD
product as they do a currently available Boeing product. Experience is hard to come by for maint. crews as they don't see enough planes to get really profficient. Not to mention the thrust reverser design on the 90 is one of the most maint. unfriendly pieces of equipment ever to make it off the drawing board. It takes two AMTs about an hour to open and close the TR
for maint. as compared to about 5 mins. for 1 person on about every other plane out there. It wouldn't be that big of a deal but they need to be opened about every other day.
I agree the 90 is a nice plane to fly on but I think we'll see most of their routes at Delta taken over by the 737-800 or 757 sometime in the not too distant future.