Other reviewers have noted that the show's first two seasons were the best, and that it made concessions to teen drama in its last seasons. I'd like to note that while the later seasons were not consistently at the level of the first two, the show never completely lost its barbed perspective, and many episodes still rose to the level established in the first two.
"Daria" is currently in re-runs on Noggin, which has decided to bowlderize it unfortunately, some of the sharpest material has been cut. It's still outstanding. Keep your fingers crossed for the DVDs.