Felicia Day turns in one of her best performances in this episode by playing Dark Charlie and Light Charlie. The concept is obvious and the script doesn't do a whole lot with it but the acting, special effects and the gags all make this a very striking episode. It feeds into the Mark of Cain story well with Dean and Ackles also turns in a touching performance. If anything I wish Dark Charlie could have been in a another episode. I also think the script gives a good resolution to Charlie's backstory. It fits in with the Winchester's history.
Any if you listen to the commentary, Day just happened to be filming the main Dark Charlie scenes on the same day some gamergaters jerks (IMDB prevents me from using the properly stringent insult here) doxxed her. Day said-and you can tell once you have heard about this-that the doxxing was used as motivation for her performance. I like the way this episode interacts with the internet Zeitgeist because of that.