I finished up my rewatch a couple of weeks back. I had seen seasons 1-4 a million times, and 5 a few, but don't think I had ever rewatched seasons 6 or 7 after they originally aired so there was definitely some stuff I didn't remember. For instance, I didn't really remember how bad and unlikable Don got in season 6. I still was on his side for most things, but man he was a complete asshole at certain times. But man oh man it was still so satisfying when he ultimately defeated Jim Cutler at the end of the first part of season 7. "The Strategy" and "Waterloo" might just be the two strongest back to back episodes the series ever did.
I also remember being initially offput by Don's little adventure at the very end of the series, but on a second watch, it made complete narrative sense. McCann was indeed a soulless "sausage factory" as Don called it back in season 3 and I can definitely see Don saying "fuck this" and getting out. In fact, I kind of hate that the final scene implies that he ultimately went back. I wish we could get some sort of epilogue season where it showed that Don went back, did the Coke thing, and ultimately left and just did something else with his life.
Overall, I think seasons 4-5 were the show at it's absolute peak, but the whole thing is a fucking masterpiece.