This film gets 5/10, because it's almost exactly one half of a perfect film.
Let's start with the good parts and get them over and done with. The artistry is outstanding. Sound design is great. Character animation is only average, but that's plainly not the creator's focus so that doesn't matter. You can write pages and pages on how painterly Shinkai's work is, how any single frame of the film can be picked out and put on the wall as a work of genuine art.
A film needs more than that to be an actual film. It needs a story, and believable characters, and this is where the film really falls down. The primary motivations, and actions of the characters may be convenient for framing the scenes Shinkai shows us, but on closer examination, simply fails to make sense.
Let's pick out one character. Okabe, the mentor to our main protagonists. We learn that he was separated from his family in Ezo by the separation, which led to him becoming a terrorist leader to fight the Union. So, his whole plan then is to strike at the Tower. Then when the heroes come to him, he lets them fly the missile and destroy the tower instead, after they show him their 'determination'.
How could a man whose entire life is lived due to the separation from his family agree to an operation which stands a high chance of destroying the entire world and by the end of the film, probably actually killed his entire family? That gigantic hole in Ezo isn't just for show. Why would he put everything into the single basket of the experimental plane built by a couple of highschoolers? Why can't they fly the UAV as well as the plane?
Look closely and the entire substance of the show is cardboard. Characters react to the start of WWII - with an unprovoked declaration of war by the US on the Union with total apathy, instead of, well, horror. The US declare war because of the Tower, but don't actually mount any direct attacks on it. The security on a classified research facility in which rests a woman whose staying asleep keeps the entire world in existence is surmounted by a single guy with a broken arm and a stolen keycard. The Union scientist who designed the tower decides for no apparent reason to make a device - at enormous expense - that will either destroy the world, or send his granddaughter into a coma, *and then* destroy the world when she dies.
Look at the frames of this film, and you're notice an interesting thing. The characters are rarely in the middle of the image. They flit in and out, hang around the peripheries, slanted or small. Sometimes their voices are silenced. Sometimes they just aren't there at all. It's like they are just guests, lucky children given a chance to play with the director's fabulous fabulous toys.
Shinkai needs a writer. Needs one badly.