Yeah, if anything he was a great casting choice. Guy barely has to act.
I still find him effective in delivering the free-market ideology, getting annoyed at the hippies and all. Some of the lines are fairly intellectual in nature and he doesn't sound too phony when delivering them.
But yeah, the role will probably stick to him for the rest of his life. I don't see him parlaying Don Draper into something bigger. That is it.