"Gantz" is well-known in the anime world as a generally violent and sex-filled show that would excite most teenagers. What makes the show worth watching is that the creators seem very much aware of this, and use it to their advantage. The result is a show that, as long as you stick with it, becomes one of the best examples of sci-fi horror anime you can find.It's surprising how unlikeable the show is from the start. Kurono is a generic high-school student who happens to be a pervert (who wasn't one when they were in high school, after all). One day, he sees a homeless man fall onto the tracks of a subway, and is urged by his good-but-tough friend Kato to get him out. The scene shows the best and worst of the show: it makes use of 3D environments with characters placed in the space, a strange appearance but they get points for trying, since the whole scene is done in one ambitious shot. Also, several on-lookers simply watch and comment on the situation, another example of social commentary. Finally, we see both Kurono and Kato get run over for their deed, their heads cutting clean off and flying through the air, in a poorly animated but still grotesque fashion.But they aren't dead. Instead, they find themselves with several other confused people in a small apartment with a big black ball. The ball, called "Gantz," tells the people they must use provided futuristic weapons and suits to hunt aliens living among society, and succeeding within a time limit will allow them to go home, until they are called upon again. This repeats until someone reaches 100 points, at which point something is supposed to happen.It ultimately feels like a satire, but one that the characters take seriously. Almost all the characters in the show, even the main protagonists, are unlikable, and we see them in this game-show like setup where they hunt down strange aliens that take odd forms. It feels immature, even when the characters die off one by one in horrific fashion, with limbs lopped off, heads exploding, and bones broken. Immaturity permuates even during the sex and rape scenes, which feel like they are there purely to cater to a certain teenage audience. Yes, the show has more violence, sex, and profanity than most others, but still feels like a kid's show at the end of the day. 
And yet, I think I liked it. The more the main characters survive, we see them change and develop as they go through both the tortures from Gantz and from everyday life. It makes for great social drama, and if you get past the halfway point, you'll want to marathon the rest. Of course, the commentary on human society is compelling, if somewhat obvious. And it brings up a ton of questions regarding the sci-fi elements, which sadly don't get answered... the ending episode is great but also ends ambiguously, and you'll want to read the manga or Wikipedia to find out the rest.The animation is considered one of Gonzo's strongest, but I don't know why. They tried using 3d environments and placed 2d characters there, and while it is ambitious, it looks weird, especially when the camera moves and zooms in to see how pixilated they appear. Otherwise, it's good but standard animation, and doesn't hold a candle to "Gankutsuou" or "RomeoXJuliet." The character designs are also somewhat ugly, not unlike real life. The violence is disgustingly good, not visually, but enough to get the point across. The music is fair, but the opening feels as juvenile as the style, and the dub leaves me wanting for certain characters.So, yeah, the show's got problems. But it also makes for a great guilty pleasure that makes you think as well as delight in the violence of it all. For its genre, you'd be hard-pressed to find better.