This is a non-spoilery blanket review of the first three editions of Highschool of the dead, since I read them so closely together.
The best thing about Highschool of the Dead was the art. It's very well illustrated with a wide range of facial expressions, well designed action scenes and convincing zombies.
From there it was pretty mundane.
The story is pretty much The Walking Dead with teenagers, I didn't find anything unique or exciting about Sato's approach to the zombie apocalypse (at least not in the first 3 volumes).
One thing that irritated me the most was the ecchi/harem comedy threaded throughout the volumes. It's not something that usually bothers me and I suppose it is some sort of comic relief, but it seemed exceptionally out of place with the seriousness of the zombie apocalypse story line.