Just got out of the theater. Initial impressions:
This was awesome! It did everything right in the same way that the 2009 Star Trek movie did, with lots of references and in-jokes, a well-designed plot, sweet animations, and an explanation for the awkwardness that didn't mess up the canon. Semi-spoiler: it's not earth, just a weird alternate dimension with humans, and they all have pony names.
But it did have the most awkward, most out of place Wilhelm scream in history.
If I had one complaint, I wanted to see more background humans that corresponded to background ponies. Where's Derpy? Where's Lyra?
I may write more later, but if you put any stock in my opinion, go see it now.