Very late to the party here, but I blew through Mad Men in about three months, and boy was that worth it. I had watched about the first two seasons (maybe) quite a while ago, but I gave up because the show just wasn't what I was expecting. In particular, I just didn't "get" Don's backstory and found that a distraction from the show I thought I was going to be watching, which I guess I thought was going to be a bunch of personal backstabbing in an ad agency in the early 1960s. Obviously the personal backstabbing is there, but with the passage of time I was able to re-set my expectations and jump back in, and it really paid off.
Having binged both Succession and Mad Men nearly back-to-back, I think there's a lot of value to binging these sorts of shows instead of dragging them out over years. These shows have a lot of character development and very subtle callbacks that would be easy to overlook if you're supposed to be remembering an episode that aired three years ago, but that are much easier to follow if you just watched that episode two weeks back. Breezy, uncomplicated shows that don't demand too much of the viewer (e.g. Game of Thrones) work just great on a weekly basis. This one was better to plow through all at once.
I had expected the finale to be more of a happy ending. Because I live in our culture and am online from time to time, I knew that the finale featured Don coming up with a particular famous ad, so I had psychologically coded that as "happy ending," when the ending was actually sort of pitch black. Big fan of that. In fact, I liked how they wrapped up every character's story line and thought everyone got a good send-off. I have no idea how this was received at the time, but I thought it was one of the better endings to a long-running show that I've seen in a while.