This film is actually pretty good. There are a lot of flaws but overall, it is enjoyable. The premise is incredibly strange, and when I first heard of it I expected it to be terrible. But the trailers seemed to have a few bright points so I decided to give the movie a shot, if mostly for curiosity's sake.
The stronger points of the film were the "fish out of water" scenes where Twilight struggles to adjust having a human body and using human tools in a human world. Another thing I really liked was the plot. With the initial announcement for the film, I feared that it would just involve "Twilight goes to the human world to learn and does high school things". But instead, the events of the film are driven by a high-stakes plot. The humor is pretty good, with the aforementioned "fish out of water" moments and some very funny visuals. The animation was bouncy, colorful, and fun.
The music is okay. I liked the lyrics but the beat and instruments were all too pop, as opposed to the more varied kinds in the show. I think the songs would've been better with different music and singing styles.
Now to the bad points... Flash Sentry is a big one. He doesn't harm the movie too much but his inclusion is completely pointless. He's a flat character and doesn't really do anything for the plot of the film. He only exists to be Twilight's love interest, but it even fails in that regard because he has far too little time for a believable romance to develop. (but hey, it's better than the romantic subplot taking up the entire movie) Another problem with the film is that it reuses too many elements from episodes of the show. Snips and Snails being lackeys to the villain, (like with Trixie) and the villain quickly reforming at the end, (like Discord). I felt like the whole "Mane 6 are no longer friends" thing was really weak; did they really not try to clear up the situation before deciding they can't be friends? I also feel like the movie would've been better if the entire Mane Cast was along for the ride. Their interactions with each other are the heart of the show.
Overall, fans of the show should give it a watch. It doesn't hold up to the same quality as some of the better episodes of the show but it's still enjoyable. If nothing else, watch it out of curiosity. It's a flawed movie but it's still a lot of fun.