As a big fan of zombie stories, I started to read Highschool of the Dead the very moment I heard about it. As a normal zombie story, HotD throws you at the middle of action right in the first pages of the manga, and never stop that atmosphere of adrenaline, horror, survival, etc, like the old Resident Evil series.
The disease is showed right where it begin spreading, on a highschool, starting the story of Takashi Komuro, a student who's trying to survive with a few friends from the same school. While they try to escape, it starts to get harder to survive, all the conflicts, situations, finding survivors, exactly like a good zombie story.
The art can be a bit strange at the beginning but you'll get used to it. One thing that I didn't like it is the ecchi scenes. And there's a lot of it. Get used to see plenty of ecchi + fanservice (I bet it's an up point for many, but I'm not a big fan).
And touching the ecchi subject comes the characters. "They're nothing outstanding, just normal", you'll might think in the first chapters, but sooner or later you'll identify yourself with one of them, as they develop themselves pretty good in the various situations that happens on the story.
As said before, if you like action, horror, survival, liked to play the old Resident Evil series in the dark (or was too scared to be in the dark) just to increase the experience and it's in doubt of HotD, give it a chance, it's very loyal to all zombies famous stories you have heard about and it's great for a quick read.