The end of Mad Men was a little ambiguous, but most of the audience seems to believe Don's "om moment" was actually just a creative breakthrough that led him back to Madison Avenue with a big idea for Coca-Cola. Ding! That smile was not one of enlightenment, it was one we've seen Don flash before, and it seems pretty obvious that this whole journey has been a carousel ride landing him back at a meeting with a knock-out pitch. However, others believe Don is still at the Esalen Institute, or something. For the less cynical amongst us, Jon Hamm is here to clear things up—he told the NY Times:

I think there probably is [one correct answer]. But I think, like most stories that we go back to, that it’s a little bit ambiguous. We had talked about this ending for a long time and that was Matt [Weiner]’s image. I was struck by the poetry of it. I didn’t know what his plans were, to get Don to this meditative, contemplative place. I just knew that he had this final image in mind.

When we find Don in that place, and this stranger relates this story of not being heard or seen or understood or appreciated, the resonance for Don was total in that moment...

My take is that, the next day, he wakes up in this beautiful place, and has this serene moment of understanding, and realizes who he is. And who he is, is an advertising man. And so, this thing comes to him. There’s a way to see it in a completely cynical way, and say, “Wow, that’s awful.” But I think that for Don, it represents some kind of understanding and comfort in this incredibly unquiet, uncomfortable life that he has led.

You can read all of Hamm's comments here—including those regarding the critics saying some of the loose ends were tied up in a rom-com package (Peggy and Stan 4 Ever, though).