I think it's underrated. It's a good, solid show that happens to be doing two things guaranteed to get it dismissed by the maximum number of people: (a) a zombie apocalypse, and (b) shameless fanservice.
1. ZOMBIE APOCALYPSE
I wouldn't call it horror. It's an action show in a super-gory apocalypse where civilisation is collapsing and life expectancy is measured in minutes. It's completely unreconstructed, mind you. It's happy to restrict itself to standard genre building blocks. Nothing here will surprise you, but it's still a well-told story that shows the full range of humanity (from "heroic" to "backstabbing scum bastard"). There's a cult. There are Best Friends Forever who, um, aren't. There are nice people in dangerous denial.
There's fanservice, yes, but most of the time this feels incidental. The focus is generally on the battle to stay alive. The first couple of episodes are particularly stark in their portrayal of the bloody on-screen deaths of almost everyone in a school.
Put it this way. This show is an exploitative boobfest that's full of panty shots... and yet apparently it might have more female fans than male ones. The girls aren't Hollywood-perfect "Strong Women", with Rei's emotional vulnerability, Saya's aggressive ego complex and Saeko's suppressed complex about her own sadism. Then there's the gargantuan-breasted Shizuka-sensei, who's such an airhead that you might wonder how she passed her medical exams. However they're all invaluable team members, with Saeko and Rei providing combat muscle, Saya the brains and Shizuka the wheels. (Shizuka actually has the highest body count of anyone, thanks to aggressive driving techniques.) Saeko's by far the most badass fighter, although the boys are no slouches either. Takashi's decisive, strong and an improviser, while fat loser Hirano is a gun nerd who's done weapons training in America and can teach everyone gun safety.
Oh, and there's no harem nonsense. The girls don't all fall in love with Takashi or anything like that, although that's not to say that there aren't relationship complications. The girls are surprisingly mature about it, though.
The zombie side of things is solid too. The show sets up rules and thinks them through. These are essentially Romero zombies, but they're very strong and you've only got a few minutes if one bites you. They're also blind and attracted by sound. Our heroes also discuss how long it's likely to be before the zombies decompose so much that they're no longer a threat. (Answer: theoretically it shouldn't be long, but then again theoretically they shouldn't be walking around in the first place.)
Apparently the manga's author is a gun nut. All the weapons here are detailed and accurate, down to the limitations of the specific guns being used, although personally I've have been trying harder to ration ammo. Apart from fanservice, this show's main flaw as a serious apocalypse drama is a willingness to prioritise "cool" over "plausible". The Ninja Bullet Time Boob Shots in ep.8 are ridiculous, as is the stunt driving in ep.12. (A more serious objection would be that the manga portrays heroically an organisation of history-denying Japanese nationalists, but fortunately the anime chose to avoid that element.)
Small-breasted women do not exist in this universe. Bosoms are only allowed to be (a) big, (b) huge or (c) so enormous that they get comedy boing sound effects. Knickers are for flashing. That said, though, the fanservice isn't usually getting in the way of the episodes. The exceptions are:
Ep.6 = the plot stops so that all the girls can get in the bath together.
Ep.7 = a continuation of ep.6, but somehow even sillier because here the "just got out of the bath and only wearing a face flannel" fanservice is at war with a serious story. (They don't get dressed until ep.8.)
Ep.13, i.e. the fifteen-minute OVA episode = a daft fanservice romp, with a punchline that arguably undercuts the threat level assumptions of the entire series. You've either got to pretend the episode didn't happen or invent fan theories about why a certain character didn't get eaten.
That's overlooking more isolated silly bits, e.g. Saeko's Boob Matrix in ep.9 (which isn't in the manga) or a topless Rei not bothering with a bra in ep.10 when the entire gang's holding a meeting around her sickbed.
This show is banned in China. I enjoyed it. I like good zombie movies and I liked this. You've got to be able to handle some silliness in what's otherwise a tough, blood-splattered story, though. To nitpick a little, I don't think the loathsome Shidou-sensei earned his story prominence. Villain points: tip-top. Plot impact: almost none, in the end. However I presume the manga had plans for him later. Actually I wouldn't mind seeing this get a second season, but unfortunately the manga's been on hiatus since 2013 and you'll have to watch Triage X if you want a more recent Shouji Satou fanservice anime. Gets extra points for using music from 28 Weeks Later in ep.1.