Have just watched the last episode and it was great. However I am going to go against the tide of opinion here and say that I can't stand Don - he is vile.
He told Betty 'You're a whore' - when she hasn't even slept with her new bloke yet, and yet he is an absolute man-whore! I know attitudes were different back then, but even so ... In fact he has treated Betty very badly in many ways. He's a neglectful parent as well, though admittedly better than Betty, with her constant "Go upstairs" the children.
Then he's been horrible to Peggy, rude to his secretaries, unjustifiably sacked Salvatore etc etc.
In real life you wouldn't want to know this guy, OK he is reasonably good looking but he's just a pig!