Have s1-s7 already on dvd and still waiting for s8. And in case you wonder why i bought s4-s7 its because it's still good entertainment besides that i like to see Holly play Piper.
But when i look objective to all seasons i have i say s1-s3 is better then s4-s7. s4 is still good since Rose is really involved in the show. Besides that Cole walks around which makes it good too. s5 its start to get down. Cole gets less involved and Alyssa is trying to get the show her way. s6 it's all about Chris and s7 all about Leo. And in both s6-s7 the main role is Phoebe.