I really liked this episode, a lot of people have given it stick for being, plotless, pandering and pure fan service. I don't see those as being bad things, this episode was pure character development, and it was a lot of fun to watch. Yes, it was blatant fan service but I think it's a good thing that they chose to make this landmark episode a tip of the hat to us, 'the other fans' of MLP.
Derpy was of course brilliant, I love her even more now than I did before. The Doctor was hilarious, and it was good to see other fan favourites like Octavia, Lyra, Bonbon and Vinyl getting a bit of airtime. We even got to see Steven Magnet again, and not least Jelly Pony appearing again after his blink and you'll miss it role in 'one bad apple'.
Aside from Derpy though, I think the character that really stole the show was Gummy with his thought-provoking monologue, so random, and so funny.