Its good but it also has its share of weaknesses. I think the story starts out really strong. Its mysterious because no one has a clue what's going on at first, with fatal consequences. It has elements of horror in the beginning because the characters never know the whole situation that they are going to get thrown into, they are powerless against Gantz and its machinations so there's a good feeling of being trapped, and anyone can die. There are alot of bad people in this series, some are supposed to be on the "good guys" side, which sometimes makes it more disturbing than what they are supposed to be fighting. On top of that, one of the main leads starts off as kind of a selfish jerk, who slowly evolves over time. All of this is decent.
Unfortunately, there are weaknesses, some plot holes, and a few things that never get explained as the series moves on. A few times, it felt like the author was stretching for ideas and some of it became filler. The story started out small but has ramped up relatively quickly, arc after arc to mega-scale confrontations. The result is that some of the character development, plot and romance aspects have taken a back seat. This is unfortunate because its become a bit shallow, and lost some of the atmosphere it had in the beginning.
Another criticism that other reviewers have made and I agree with is the artwork. The characters look good and the art style has matured in contrast to early Gantz and certainly since the mangaka's earlier work Hen. However, the backgrounds are sometimes terrible. Its obvious that some are photos that have been run thru some Photoshop-type filter and it doesn't match the artwork well.
I still like Gantz and I am sticking with it, hoping that the writing will turn its focus back on the characters and resolve some of the many issues that have been left hanging.