It was pretty good. It wasn't amazing or the best finale ever, but... it was fun. ^^
I sorta object to Discord being so easily tricked. He should be smarter then that.
Well, I expected him to explain at the end of the episode that it was the same plot as in the season opening: make Twilight be the one who confronts the danger so she'll grow as a Princess. That wink to Celestia might be an indication of them having an understanding.
I kind of liked how even Celestia admitted that she had no purpose for Twilight yet. So far her modus operandi seems to be: find potential threat to Equestria, like a pegasus with mind control abilities, and then make them live the dream in order to have one more agent of Harmony.
To be honest, the last thing I expected Twilight to be Princess of was Friendship. We already have one of those: the Princess of Love. Cadence supposedly ruled over emotions, so I thought Twilight would have been ruler of the mind: Magic, Leadership, Harmony, even Earth.
Princess of Mind would have made for a nice synchronicity with Emotion / Love, just like how Sun and Moon go together.