As per custom, it's required for me to talk about the ending, even if it's right at the start of the review. In the form of a poem I heard long ago; It sucks. I've heard how bad it was, and had a choice of leaving Gantz with fond memories of the second-to-last arc's ending and heading to the manga or forging on despite warnings of the anime's ending. I choose the second route and got the bad ending with no save file to go back and take the route to the good ending.
Ranting aside, the rest of the story is quite good. It starts off nicely, continues until it hits a moral brick wall and forges on into another moral brick wall and keeps on running headfirst into them. It may sound bad, but that's what helped drive it forward for me; how it deplicts such mature content and a lack of morals in some of the events. It doesn't shy away from the sex or violence and at times embraces it to some disturbing entertainment to the viewer (To some of them at least). This world is neither friendly nor kind and that's for all the better. But even with all the mature content, it can move quite slow at times, in particular when the characters are waiting for Gantz to introduce a new mission. It was a love/hate thing, as the waiting parts had the viewer get to know some of the characters a bit more.
There's this one part in the anime, early on, where Kei (Kurono) leaps off the top of a set of stairs, dozens of stairs, into the air and the camera shows the ground coming towards him, gaining speed as he looks back. For some reason, I really loved that part. Always wanted to do something like that, without having to worry about the impact. Not to say that the other parts don't look good, as they do. And some of the aliens are pretty cool. More so for one in the second to last arc: You'll know it when you see it.
The OP shows that small event I described and with the actual opening to it makes it seem even cooler. And overall, the OP is... strange. But it's a nice strange, as I found it oddly enjoyable despite it's craziness. The EP is a bit sobering, in comparison to some of the chaos that can ensue in the anime. As a final note, that song that plays at the start of a debriefing... quite the contrast to the events to come. For VO, watched it in dub with some sub mixed in and enjoyed it, don't have much to say about it apart from that it worked for me.
Kurono... my first thought was; "What a douchebag." After a while: "He is such a douchebag." And then farther on, way farther; "... now that's a plot twist!" Simply put, I have to agree with a lot of others on hating Kurono Kei Kurono. Unlike his friend, he is- bah... moving on, his friend Masaru Kato is the epitome of a nice guy. This guy wants everybody to live, very rarely has a bad thing to say about someone (Vulgar wise, or overall) and can be considered as the true protagonist as opposed to Kurono, whose thoughts we more often than not hear. The other main character is Kei Kishimoto, the girl who provides most of the fan-service and sex factor. On the outside they may seem generic but as the series progresses, we get to see more into them; more of their past, how they ended up here and some of their flaws and imperfections. Not a single person in this series is perfect, far from. It's all the better that they aren't simply meat puppets but human beings, abeit only some of them have decent depth.
Even with the ending, it's definitely worth a shot, even with the slow moments where it feels like it's just wasting time. It messes around with morals and has its fair share of violence; it's mature, doesn't tone down the violence and is just shy from going completely overboard with the ecchi. If you can stomach the content, then have fun; just beware of the ending.
(Can't really come up with a final rating I'm content with, so I just gave it one inbetween what I thought worked best; between 8 and 8.5)