Gantz is not a fantastic anime. It is likely not even a good anime. But it may be an enjoyable anime if you come into it with tempered expectations and a willingness to engage with its characters, not the characters you wish it it had.

I finished it. I've got a personal policy about not finishing any anime unless:

• I can give it a 70 or better
• Some drastic swerve in the last few episodes sours my enjoyment of the rest of the series.
• It's a dedicated hate watch that I've committed to

Gantz falls into the first category if only barely.

Good/Enjoyable

The protagonist, Kei Kurono, is a bad person. He's whiny, selfish, lazy, immature, uncontrollably horny, misanthropic, and desperate for validation and affection from the very people he hates. He gets humiliated by his classmates because he can't stop thinking about titties, but then turns that humiliation into impotent rage when he can't lash out at his classmates and teacher.

He vacillates between complaining about everyone around him and also wondering why no one looks up to him or likes him. He wants to score with a hot girl but has never ever considered how to do so or what women like or how he can be someone that others want to be around.

His English voice actor captures that dynamic perfectly, and every obscene insult and ineffectual complaint sounds exactly like you'd think it would, both when thought and spoken.

Spoiler, click to view

Thankfully, he is shamed into being a decent person by other members of the cast, and improves over the course of the series. It's never enough to make him a good person, but

Thankfully he gets less insufferable throughout the series.

The remaining cast pivot around him, each acting as obstacles, goals, or foils. Kishimoto and Katou are the woman he can't have or impress, and the more moral, more charismatic, less vicious leader that he alternatively envies and despises. Nishi is the actually brooding and vicious teen that forces Kei to confront his own morality and ethics.

Kei is full of contradictions, and by following him and his thoughts, the show forces the viewer to listen to all of them. It's entertaining.

Bad/Unenjoyable

The pacing is poor, the action comes in fits and spurts, and much of the show depicts characters yelling at each other while monsters lumber towards them. If this is an attempt to show the worst of humanity thrust into an impossible situation, it's understandable but unless you're deeply committed to the characters and their points of view, expect to spend time wondering why won't [character] just do something?

The 13 episodes cover 2 missions that each could have easily been 1-2 episodes shorter if not for the plodding pacing

It's sole female with speaking lines on the hunter team has a sever case of Sakura-itis. She doesn't accomplish much, and the other characters sometime comment on it. But she reads as depressed and scared, and is finally, finally roused into meaningful action late in the series. It's a pity that before that she serves to mostly highlight exactly why Kei has never slept with a woman before.

Kishimoto is very busty compared to most anime designs, which in a vaccum would be a massive plus for me. The show makes sure to put her bare assets and nude body on display every chance it gets. This ends up in the "negative" column when she reminds Kei (and the viewer) that she's still a high schooler. I prefer my anime fanservice to come from women aware of their sex appeal, not frightened, disaffected high school girls.

Conclusion

The show is... fine. It's grim. People die in graphic and grotesque ways. The pacing is slow.
But the characters are interesting, if not compelling. The way everyone relates to Kei is a fascinating kind of character study.

If you ever wanted to watch a dumber Light Yagami fumble his way through a death game, you could do a lot worse than Gantz S1