I finished the game with a translation patch a while ago. I loved it.
I think it treads a fine line between realism and fantasy. There's quite a few things that are obviously more than absurd in a real life setting, but it's absurdities are consistent with the overall style and mood of the game. It's over the top, but it never becomes overBEARing with its ridiculous moments.
I do think that the characters aren't overly interesting. There's a sense of character development to each of them, but they seem to fulfill a very specific role throughout the whole game, depending on what they are branded as. I personally didn't relate nor empathize with any of them, but that didn't stop me from enjoying the interactions between the characters.
To me, the main points of interest was the premise (people having to kill each other in a Battle Royale like setting), the zany presentation and the mystery of it on both micro and macro level. The main theme wasn't anything special, but I commend the developers' concluding the game in such a tactful way. If you're into any of those things I mentioned, this is the game for you.
All in all, it was a very enjoyable ride, but I can see why it might turn off some people. The only gripe I had about the game was the typical illusion of choice, albeit in this particular case it was downright insulting.
I'll probably expand on my thoughts later, it's just that my head's going nuts after being in the sun all day long.
Kudos to Project Zetsubou for translating it. They took some liberties, but it was worth it. The translation screams quality.
Also, from what I saw, anime sucks.