This isn't really one fic though, currently it's 3. The first is about Derpy (Their usual 5th player) being busy, so they get Applejack to replace her. She mocks them at first, but gets into it and has fun. The second is several months later. When Derpy found out they were playing without her, her mind snapped and she began to think she was her character. She then did something, which involved hurt beavers. Now they've all given up the game, that is until the Cutie Mark Crusaders find the game books in the library and want to try it. Each of the Mane 6 tries their hand at being DM with Spike gone, and they all fail in different ways (Each being how a player of that type usually does suck at DMing). The third is several months after that. Applejack has gone from a newb to a Min-Maxing pro, and the girls are worried she'll get too obsessed and loose her grip on reality like Derpy did. The third is currently in-progress.
So how good is it? To me, amazing is the word for it. It nails all of the characters and how they would play D&D, as well as how they would DM given the chance. It very much hits what D&D can be like as well, and ranks up there with the Community episode Advanced Dungeons & Dragons and The Gamers 2: Dorkness Rising as a great example of how D&D is actually done. What I really love though is the jokes. This thing had me laughing the whole ray through, and I haven't read a funnier fan fiction since...Last Words the Dresden Files crackfic, I would say. Overall, this is a huge recommendation to Bronies who like D&D, non-Bronies who like D&D, and Bronies who've never rolled a D20 in their lives. Laters.