I came to AB! expecting a tearjerker, but too often I felt that what I was being asked to feel was too shallow or cheap. I don't like the feeling of being 'worked' by an author, whether it's anime or something else. One of the things that I think AB! really suffers from is the demand for immediate empathy, handing you tragic backstory after tragic backstory. At its best, I think this show does well bordering the line between a baseline sense of tragedy and the sort of sweet, silly or absurd situations that could arise with a bunch of pissed-off young people living in an eternal high school. At worst, it's messy and confusing as it sidelines worldbuilding and character depth for one-off action episodes and overwrought emotional conflicts.
The basic premise - the unsure do-gooder caught in a conflict and world he doesn't understand - has a lot of potential. The eerie shots of empty classrooms and the endless repetition of this sort of nether-world are enigmatic and may delight the curious - but to claim, as the summary on this site does, that this is a story unraveling 'the mystery of the afterlife' misses the mark a bit. For the bulk of the show, and where it's at its best, it's primarily an episodic, slice-of-life story about unhappy characters rebelling against meaningless suffering and a faceless higher power through simple antics. It's the bonds that form between these characters, and their growth as characters, that form the strongest element of the show and the one that may have you in the tears you've been promised by the end.
Unfortunately, these characters are skating on the rather thin ice of the world they find themselves in - it also doesn't help that for all the stronger, interesting characters, they're also loaded down with the predictable cast of tropes that add a few laughs but exist by and large to shoot a gun, drop a quip or two, and then disappear. By the end, the 'unraveling' that we've done leads to more questions than answers about the world in which they exist.
While the show has a tendency of taking itself too seriously, I enjoyed watching these characters interact most of the time, and they'll be what sticks with you after you've wrapped up your time in this very strange world. When they're being playful there are some very funny moments, giving rise to situations with just the right mix of sweetness and sadness that I felt most deeply in watching this show.
Don't come to this expecting a masterpiece, but if you take it for what it is, it's probably worth the 6.5 hours.