I LOVED it! I found myself smiling for most of the movie. The fan service and jokes relating back to the episodes (Pinkie's psychic abilities, SpikexRarity, the exact recreation of Twi and Flutters original meeting back in Ponyville) were abundant, but not overboard. I also liked Trixie's peanut butter cracker addiction and the CMC posting their song on YouTube.
Overall, the plot was great. It established the "united we stand" theme that the show's been going at since it began. I liked the
Quantum Leap style dimension travel, where all of Twi's friends exist in the other dimension, yet they don't know her. From the trailer I was thinking that all of the mane six went in the portal with Twilight, but the way they did it made it better. Sunset Shimmer, although she appeared to be the "school bully," she was actually a really well-developed character. It made sense that she tried to take over human world after Celestia wouldn't let her rule Equestria. The ending "fight" scene gave me goosebumps; when Sunset shimmer turned into a demon it was freaking scary (in a good way), and certainly unexpected for a movie with a "girly" aura.
The animation was same as always, they kept the same general faces despite having new bodies. It was easy to believe they were the same lovable ponies, as they even maintained their personalities they had from the original show.
One more thing, the songs. They were good, but nothing special that caught my interest.
Overall, I give it... a decent 13.5 out of 10. It was a really well done movie that shattered my expectations.