Don't get me wrong, Gantz is a good series, but there are several things that pissed me off about it.
1. The fight in the temple, where the entire established cast bar one is cheerfully annihilated, including the coolest character in the series.
2. The fight scenes are epically long. The Dinosaur fight for example took almost 30 chapters to complete. It was a cool fight, but it dragged after a while. I actually tend to prefer the parts where fights aren't going on, as the plot and characters get their chance to shine. The fights have also lost the sheer, visceral horror that made the first couple of them so tense and fun to read.
3. Apart from the introduction of the vampires, which I'll get to in a minute, the plot really hasn't gone anywhere since the temple fight. I'm 167 chapters in and I still have no clue about any of the forces at work in the series. New mysteries have presented themselves, but none of the old ones have been resolved, in fact they seem to have been ignored entirely. This is unsatisfying to say the least.
4. The introduction of katana wielding vampires. Some people just might say that that is a little bit cliched.
5. The relationships between characters are developing nicely, but I still want some of the old characters back, especially Kato. The story is on the back-burner.
Still, the series is somewhat addictive, so I'll stick with it for now.