Please note: there will be SPOILERS in this review. Probably nothing a fan couldn't guess, but I have to touch on the ending a little bit just to encapsulate the general experience.
When the lights came up at the end of the world premiere of My Little Pony Equestria Girls, following a brief in-credits easter egg for fans, my screening companion (whom regular commenters will know as Kyle "SlyDante" LeClair) turned to me and said, "On behalf of all Friendship is Magic fans, I would like to apologize to you for the preceding movie" (this was what he meant by taking a bullet yesterday). Considering the zeal with which the little-girl audience responded to the film's musical numbers with well-prompted clapping led by festival volunteers, I suspect the official target audience won't mind too much. But when it comes to Bronies, the adult male fans of the cartoon who insist it's a well-written show...I predict there may be some gnashing of teeth. Whatever clever concepts the show may have - and I cannot claim to be especially familiar with it - the movie does not do very much with (there is a funny parody of political attack ads, but that's about it). And for a TV cartoon turned movie (think of how Simpsons did it, or Beavis and Butt-head), there was no extra expenditure to make things look better on the big screen. Given the relatively simple nature of the drawings, you could probably watch this on your phone and not lose much visual detail, if any. I wouldn't suggest paying to watch it, but if you held a gun to my head and told me I had to see it again, I'd say, "Sure, fine whatevs." It's far from the worst movie based on a TV cartoon I've ever seen. If you can really call it a movie at all.
Things start off in Pony World, Equestria, which looks a bit like what might happen if Jon Kricfalusi tried to illustrate Dr. Seuss and it was all ponies. Purple pony Twilight Sparkle (please tell me this name is not derived from that thing Edward Cullen does in sunlight) is attending a Princess Summit, having recently received wings and a crown with a magic gem in it. She's apprehensive - worried about the responsibility of being a leader - and unable to find a comfortable sleeping position for the new appendages. But no sooner does she briefly manage some slumber than an evil pony named Sunset Shimmer - a Darth Vader-like failed student of Queen Celestia who fell to the dark side - steals the crown, and absconds with it through a magic mirror into another dimension. Twilight must follow and get the crown back, but the doorway will only be open for a limited time, and she must go alone. Well, but Spike the dragon goes with her anyway, so scratch that rule.
If you remember the TV show Sliders, it pretty much sets the template for what happens next. Twilight enters a parallel reality in which she is now human (sort of - humans in this realm can be blue and pink, for example, so it's not exactly our reality) and there are human versions of every other major character on the show, all with the same names (presumably, to be consistent, there must already be a Twilight Sparkle in this dimension, unless Quantum Leap rules are in effect). So she shows up at a high school, just starts pretending she's a new student, and everyone, including the school administration, totally just accepts it. Sunset Shimmer is the queen bitch of this new school, having become so by dividing friends against each other - she sends nasty fake emails, and everyone just totally believes them, which makes them dumb enough to deserve the leadership they get, if you ask me. But Twilight's smarter than that, even if she does keep forgetting she's not a horse any more.
By the time we get to a point where there are zombies and demons, the girls are now half-animal hybrids for no reason, and they're blasting rainbows at the big bad, I had given up hoping for consistency. Since the early days of Care Bears I've accepted that many toy-based cartoons aimed at girls will end with unexplained powers somehow saving the day because magic, but that doesn't make it any more...um...believable? Not really the right word. But while I'm at it, how come nobody else jumps through that totally wide-open portal to Equestria in the school's front yard? And why does a major pony character ask, "What are hands?" when Spike pretty clearly has hands? And what's with Spike's creepy crush on Rarity, which manifests itself as alt-reality dog-Spike wanted to be petted and scratched by human Rarity all the time? (If it's a nod to the more deviant fans of the property, I get it.)