Season six was indeed a nightmare but it's actually the show's highest rated season. Go figure.
I think the biggest mistake they made with the new characters was trying to make them carbon copies of the characters they were replacing. Alison was supposed to be the new Suzanne but she did not have nearly the charm that Suzanne had. I don't know if it was the writing or an acting choice by Julia Duffy, but I couldn't find anything likable about Alison, she just came across as mean and not someone I enjoyed watching. Carlene was annoying at first but thankfully they calmed her down and she became much more tolerable as season six went on. I'm mixed on BJ, I liked her overall but between her, Julia, and season seven Mary Jo always ranting about something, it was like having three Julias, it was a bit much.
However, even though I dislike seasons six and seven immensely, season six did give me one of my favorite episodes: "Julia and Mary Jo Get Stuck Under A Bed". It was the episode that felt closest to earlier season and just overall hilarious. "Jellies of the world" "Oh Chuck, Oh Chuck, Oh Chuck", just hilarious from beginning to end.