First of all, less than a minute into the show and Pinkie Pie makes me laugh with the Transformers reference.
Then showing that musicals are a case of "style and plot segregation" in that Twilight has trouble flying with her wings... and trouble adjusting to them period.
Oh, and her awkwardness in trying to adjust to being a princess as well.
Sunset Shimmer was actually a fairly interesting villain, albeit she felt like a bitchier Trixie, so she's not exactly original in that regard. Though the fact that she's basically a Nega Twilight Sparkle makes it understandable since Trixie was Twilight's opposite in Equestria proper anyways. I imagine Canterlot High Trixie was actually a sarcastic, introverted bookworm all things considered.
Also, I am TOTALLY okay with Spike being a dog. Why? Well look at what he is in the series proper. He's a dragon who walks on two legs and has hands in a world of ponies that walk on all fours. In Canterlot High, he and Twilight more or less switch stances. Interesting to note that Spike takes to being on all fours a lot more naturally than Twilight takes to being on two legs.
The plot made it pretty clear early on that Sunset Shimmer wanted to wreak magical havoc in both Canterlot High and Equestria right from the start, so Demon!Shimmer isn't exactly a surprise in the least.
Watching Twilight adjust to being human was hilarious and made all the better when she finally got the hang of it.
... Sorta. Also it was kinda cool how she managed to re-connect the Mane 5 in Canterlot High by simply getting them to start voicing their concerns with each other which happened by giving them a reason to actually endure each others' presence. That's right, Twilight Sparkle merely EXISTING fixed their friendship. Also a case of Nice Job Fixing It Villain since if Sunset Shimmer hadn't done that mudslinging campaign...
Also...
PRETTY PONY POWAAAAAAAH MAAAKE UPPP!!!
Don't tell me you didn't think the same thing.
All in all, I enjoyed it. It honestly felt something like a season premiere/finale all rolled up into the length of three pony episodes. I wouldn't have gone to the theaters to see this, but then I didn't go to the theaters to see this. Then again, that doesn't mean much because very few movies I want to see, I actually see in theaters. It's just cheaper to wait for them to come out on DVD, and they last longer.
Now, my criticisms.
First of all, I cannot deny it. Sunset Shimmer asking for forgiveness felt very... forced. Why was she crying? What was she sad about? I could understand if she was sad about losing but that didn't seem to be it.
...And that's actually pretty much it. That was the only problem I had with it, other than what I mentioned about Sunset Shimmer being a bitchier Trixie.
And yes, I know Sunset Shimmer is actually from Equestria, but I still consider her a Nega Twilight.