Here's a breakdown of what it might be for you:
Happy-happy:
You'll probably only like this for the first volume, skip the second volume or so, and then go straight for the last volume. Case in point: It's a waste of your time.
Dark:
You'll also not like this book from beginning to end, unless you're into the psychological spin inside of it (and it doesn't get too deep into it as some of the more readily available philosophy books out there, yet it is still thought provoking). Gore-wise... it doesn't have as much gore as you might expect from series about hordes of Japanese civilians going crazy from lack of light, order, and internet. Once you have exhausted other series like (the first few volumes of) Death Note, then you can come back over here to this manga to vent your frustration.
Normal (and I mean, those who at that particular moment, just choose to lay back and enjoy a partly dark psychological manga):
It's nice, and the characters don't get attached to you that much, so you can enjoy a peaceful night's sleep knowing that even if all of them die, you won't even flinch (hopefully). That's a lie by the way, since they actually do stay with you, just not too much that you'd be a fan of a group of senile juniors and wise old folks. It's a blast to read from the get-go, so don't try to read a volume per night since it wont scare you at all (that much).
Also, just like the other users have pointed out, there is good plot, development, and among other things, love (or lust, or whatever you would call two borderline high schoolers, alone, by themselves, and no other girls/guys left to consider).
Divinewind