Peggy is fantastic but I'm going to stick up for January Jones--I think her acting is excellent in Mad Men. Much of the reason people hate her so much is because she's so effective at playing the spoiled, self-centred ***** Betty.
However, isn't it easier to play souch a terrible person as a character? She adds no complexity to Betty. With Don, though I think he's contemptible (if you didn't notice) he's still a very compelling and I enjoy seeing him on screen. Since season 1, I can't say the same about Betty.
I guess you could blame it on the writers too, though.