It's a typical Ghibli movie -- go see it. But there's also a lot of backstory that is never shown or explained, and I find that mildly vexing. I think, story-wise, it's one of the weaker offerings in recent years. There's some preaching about ecology there, but luckily it's mild and contained to the first part of the movie.
It did remind me of the short film we saw in the Ghibli museum last year, 'Kujiratori' -- about a group of boys pretending to go on a whale hunt.