Coming from someone who's first book series he could read on his own was Captain Underpants (explains a lot, doesn't it?), this is the film I wanted thirteen years ago and its tardiness is really my only complaint. In a whiz-bang 85 minutes, this film adaptation is one that that knows how to be a sweet parable on childhood, a great-looking animated film, and a charmingly silly commitment to its source material.
2-3. The script is really messy, there's not a lot of differentiation between George and Harold, Mr. Krupp seems to come around to the boys too quickly, and there are a number of tiny issues that crop up. But for once, the potty humor on display feels close to the heart of the property rather than being forced, like with other kid-properties. And it displays some real eye-popping visual invention.
From the design of the movie to about 50% of the writing it's faithful to the source material, the other half is completely forgettable if you're older than 8 and even then you'd rapidly forget. The writers had the privileged position to spice up the childish tone of the book with the nonsensical mainstream humor of "Steven's Universe", without it feeling forced, but they didn't feel inspired as it seems.