Pros: Who wouldn't want to travel the world with someone who looks like Christina Hendricks? More importantly, though, throughout "Mad Men," Joan has been shown to be the consummate professional, someone who rocks any task put in front of her, whether it's a job she's been doing for years or something she's never tried before. She'd charm the hell out of the locals in every country, have insane airport-fu, be tons of funs at the Pit Stops, and force Phil Keoghan's eyebrow into contortions never imagined thus far.
Cons: Can be judgmental and unforgiving of those less competent than her - or who approach tasks in ways she wouldn't - which could make her a pain on Roadblocks she doesn't run. Also not sure what the passport situation would be to bring her from 1965 to today.