I still enjoyed Season 5 (4 was so-so, the characters were mostly written well, I just felt the humor was more miss than hit) because it gave good send offs for Pierce and Troy. Duncan finally came back and was even given more depth while still being hilarious, and Jonathan Banks' character Hickey was fairly good.
Then Season 6 starts and Shirley is just gone. No send off, just a few lines to say what happened to her and that's it. And Duncan and Hickey are just vanished with zero explanation.
Then they bring in the new girl and I just wasn't feeling her at all. And then Keith David, who is an awesome actor, but he felt extremely shoehorned in. I gave it three episodes but yeah I was done. To just write off a main character who had been there since the beginning so casually, and then remove one new and one returning character and replace them all with yet two more new characters, my patience was up. The Study Group was a family by Season 2, and at least Pierce and Troy were given entire episodes dedicated to their departure, and Duncan and Hickey were slowly feeling like part of it as well, but then to just throw a bunch more out like that killed the family feel for me.
It wasn't final season of 70's Show level of bad (because who can top that motherfucker who tried to essentially replace Eric in every way possible, don't even feel like checking his name I hated him so much), but still pretty weak of what I saw.
For me:
Seasons 1-3 10/10
Season 5 9/10
Season 4 7/10
What I saw of Season 6 4/10
I've moved on from hoping it will happen, but if it did hopefully the movie can reunite everyone (even Pierce could be in flashbacks or a vision or something) and give a great conclusion. Only supporting character I know sadly won't be with us is Leonard as I heard his actor passed away a few years back. RIP.