I'm gonna try and be short and simple. Skip ahead if you want.
Story: It's been done, so it wasn't OMG WOW SO CAPTIVATING!! Girl and guy from different worlds meet, there's some weird bad guy with his own agenda after the girl, some sort of lame and totally misguided thinking behind the need for the separation of said two worlds, there are competing love interests, dead parental figures, etc. It's actually fairly similar in plot to Tenkuu no Shiro Laputa (Castle in the Sky). The only difference is the back story of how people became inverted. While this is the most interesting aspect of the story, it's a little lacking, in my opinion. Some small details aren't explained (if you were thinking about it critically, you would be wondering, if Eiji's sky was actually the ground for Patema, then where were the real stars? how does the sun come up, and from where? how can such a seemingly advanced society (Eiji's) have such a flawed understanding of outer space that they didn't notice that, OH, the sky is actually the ground. and if we only see birds for the first time in the last scene of the "real" earth's surface, assuming it is earth, then where and how did they eat meat/protein/dairy?!? .....just, you know, some small observations....). 7.5/10
Art: I have nothing to complain about. It wasn't ugly, it was very fluid and consistent, and it wasn't so lazy as I've seen other animes to be where no one seems to move like real anatomically correct human beings that bend at the knee when they walk. I'm just talking about the moving characters. The background art, like the sky, well, DAMN. That really takes your breath away, especially when you see it from the perspective of being upside down. Scaaaary. They do a good job at really making you feel the vastness of the sky, which I haven't seen before in other movies. 8/10
Character: I dunno. Eiji changes a bit. Kudos. Patema is cool. Patos is funny. Gramps is the typical old man. Bad man is typical bad man. Bad man's right hand man is the same, typical. Similar to story/plot, nothing new, nothing outstanding. No amazing character development. But, you know, I didn't hate any of them. Except the ones that needed to be hated, like the bad guy. But that goes without saying. 7/10
Sound: Y'know, I didn't really pay attention. Seemed ok to me, wasn't jarring to the ears or anything. But, that could mean that it was effective in producing the right kind of ambiance. So there. 9/10
So I'm giving this a 9/10 simply because above all else, the simplicity of the concept was so well executed. There.
