I downloaded this anime after reading a review of GANTZ, stating that this would also be pretty messed up. In the end I can agree, but it takes a while to see why the comparison was made. Not that I was really waiting for that but... I guess this whole anime is just unexpected. And after watchin it, I really do wonder why they haven't made a second season or why they hadn't waited for the manga to finish, because there is no end to it. It's really REALLY frustrating, because I feel like we are only halfway through the story.
So what I recommend you to do if you're interested in this, is what I am going to do.
Read the manga.
I just looked up a few pages to see if it would be much different, and it is! So chances are quite a bit larger there will be a proper finish to the story as well.