If you are browsing for volume two, then you are already likely sold on the idea of Daisuke and Shoji Sato's zombie apocalypse. However, if you're wondering whether or not investing into the series will be worth it or not, try not to listen to the critics too much...
Whereas it's true that you cannot expect gold to be dripping off of a zombie genre's plot sequence, High school of the Dead moves the plot around in a believable sense- making the characters feel extremely human even in such impossible circumstances. Whether it's finding their first gun (something that's a little more important in Japan) fending off crazed survivors or rescuing a helpless little girl, there's a decent amount of action to be expected.
Another point of criticism would be the character development, which comes together a little more during this volume. I always operate with the idea that in a series where the characters can be eaten in the next panel what little is said between them is more important than if they could spend an infinite amount of time in a care free environment. Either way, Volume 2 had the characters make large steps in making me want them to live.
And finally, the fan service. It continues in this volume, of course, and it is actually home to the most fan serviced scenes in the entire series. However, the art and undeniable feeling in the pages were enough for me to overlook the fan service (which really isn't that bad if you watched the anime adaptation).