Not sure how I feel, honestly. It was a great episode, but I feel like it was almost too neat. Everyone but Betty came out great. That seems odd/improbable given the previous 7 seasons. Also, way too much Joan.
And I don't think it's obvious that Don went back and created the Coke commercial. I think there are two equally likely scenarios: 1) Don did go back and write it, because his "enlightenment" amounted to being good at selling stuff again, i.e. he realizes that is who he is and accepts that for the first time or 2) It was an ironic bit correlating Don's real, new found happiness with drinking a Coke, which is what the original ad was saying. Could be Weiner making a point about the entire ad business being built on using real, deep emotions to hock sodas and whatnot and Don finally coming out of that.
I personally lean towards the second explanation.