I really enjoy Mad Men. It's just incredibly entertaining.
I also can't really think of a way to describe what the show is about.
I mean, I know the characters. I know everything they do, and their relationships. But have you ever tried to explain Mad Men to someone that doesn't watch the show these days? Season One you could probably have said "It's about Madison Avenue ad executives during the 60's." Now though... that doesn't even come close to summing up what the show is or who the characters are. That element is still there, of course, but it's mostly a backdrop now. The show is about the characters, and pinning the description on where they work really does no justice to all the other tangents the show explores.
The best I can do is say "It's a show about people. And shit happens to them." Sometimes it's meaningful shit and really drives the "plot"... but most of the time it's just stuff happening. That's life though, isn't it? I guess that's the point... people may have goals and aspirations, but 90% of the time it's just seemingly random shit happening. As such it's entirely too complex to, in my opinion, adequately describe. You can no longer even say it's a show about one person, Don Draper, because so many interesting characters (Peggy, Betty) have split off in their own, completely unrelated direction.
And don't get me wrong, it's really fun to watch. The characters are great, and I'm just as interested now in the sixth season as I was in the first. But where some shows have a goal... an overall plot that you know you're constantly working towards while stuff happens (X-Files had Mulder searching for "The Truth", Battlestar Galactica had the humans searching for earth and trying to survive, Lost had the survivors trying to get off the island), Mad Men just... is. It exists, and it keeps moving forward towards who the fuck knows. It's a great ride so far, though.
The writers are generally pretty clever, and there's often a lot of subtext in any given scene. What generally looks like just something of a random occurence or non-sequitor usually ends up informing the audience about a character on a deeper level.
Sometimes, though, I have to question if they just throw some random shit in there with no deeper meaning just to fuck with us.
If you don't watch the show, I'd definitely recommend it, even if I can't really tell you more than "It's about some people."
And stuff happens.