Gantz has been one of my favorite manga for the past six years or so, and the Dark Horse releases have garnered no complaints from me. It's a seinen series which is very character-driven, with incredibly intense fight scenes contrasted by slice-of-life interactions. This may not be the series for everyone, as it certainly displays a great deal of cynicism with regards to humanity and chauvinistic behavior - as well as a great deal of fanservice.
There are many different stories contained within the series, and no two battles they fight will be fought in the same manner. People die during the battles, sometimes a lot of people die. The characters will try to get through their daily lives in contrast to the life-threatening situations they encounter through the Gantz system, and they can be rather strange lives. Every new part of the series has left me eagerly waiting for more, even when I scratch my head in confusion or surprise.
The Gantz system brings a group of random people together to fight enemies. The catch is that only people who just died are brought in. Weapons and equipment are provided, and the participants are teleported to an area around the target. They are then given a time limit to capture or kill the enemies.
Kurono Kei is your typical blank slate protagonist. He's just a regular guy who does alright at school, is a little bit of a pervert, and hasn't had much happen to him. His reunion with an old friend (Kato) leads to them getting hit by a train and sent to the Gantz room. This is where the one quality about Kurono comes out - his overwhelming desire to live.
By the time you get through Vol. 3, you'll know whether or not this series is for you. Violence, sex, hope, death, love, selfishness, and a burning desire to stay alive - it's all in here, just don't expect it all to be as pretty and straightforward as the pin-up art provided throughout. If you progress up to ~Vol 10 you'll have reached the splitting point between the Gantz manga and anime.