Games are always fun, specially when they involve mayhem and aliens. Coming out almost at the same time as Battle Royale, Gantz is one of those types of stories that we are all well familiar nowadays, that have our protagonist trapped in some sort of twisted survival game along with some other unlucky bastards and their lives are on the line at all times so they must develop survival skills in order to keep breathing, just to play some more later.

So good things first. The action is extraordinary. One of the best I've ever seen. And not because it is extremely frenetic (although it kinda turns out this way later on), but because we can feel the lives of the players at stake. Gantz is (in)famous for its explicit portrayal of violence, but I have to admit it works really well here. Witnessing such horrific ways to go (twice if you read the synopsis) makes the characters realize how fragile they really are and how that affects their psychological states. No one is safe in Gantz, and although the players have ways to defend themselves, you can always feel the danger lurking in the corners, it's extremely unconfortable but it feels realistic. It is easy to be turned off by the extremely violent, nihilistic approach towards human nature Oku presents and the gratuitous nudity, but once you get used to it, it becomes immensely addictive, making you even feel guilty by the numbness to gore that comes after the few chapters.
The art is also a highlight. While some readers may be dissatisfied with the computer made backgrounds, you cannot deny Oku's tremendous talent for action sequences, human anatomy and facial expressions.

But I still have to say, Gantz is far from perfect. The characters are extremely unlikable and it's hard to root for any of them, specially in a case where caring about them is very necessary (it's a game anyway, you need to want the character to survive) even if some of them are somehow well developed.
The story quality also suffers considerably when we get to the final arc. All the "realism" established in the beggining simply vanishes and you start to get the "shonen tropes" feeling lingering in your mouth. Add this up with some half-baked contradictory philosophy and you start to wonder why the author didn't quit while he was ahead.

Gantz is a sci fi/action manga. Not a very "scientific" one, but fun and well done (at least artistically) nonetheless.