I watched the series itself some time ago, but am only now winding down making my way through the commentaries and special features (kudos to TPTB for making a feature-rich disc collection, though it's disappointing that after a certain point the commentaries become more limited in scope). It was a heck of a show, and gave me some distressing insights into the America of the 1960s.
I agree that Sal getting a bum deal is one of my pain points, though I also guess in a way that the last scene that shows him tells us, in a sense, all we need to know. But still, he seemed like one of the less skeezy guys in the series, and I miss him (I'll also be perpetually sad about Lane). Don's firing of him and Harry's part in events were, for me, low points for both characters (though really, in a way Harry falls the farthest of anyone over the run of the series).
After having seen Vincent Karheiser playing Connor in "Angel", he was kind of a revelation as Pete Campbell here.
What's kind of interesting to me is that I'm not sure there were any characters in the series whom it could be said I genuinely liked. Peggy probably came closest, but she has a lot of not-great moments of her own. They were all fascinating to watch, though.