A friend once lent me what he described as essentially the first couple arcs of the Gantz manga, and I wouldn't recommend them. That goes double if you're like me and aren't really a fan of the stereotypical angst/fanservice/gore trifecta that those of us born prior to 1985 tend to associate with any anime/manga other than Sailor Moon, Dragon Ball or Speed Racer.* What little I read of it kinda went like this: Angst, gore, tits, angst, gore, dude fucks Lara Croft look-a-like for no reason, angst, gore, angst, fin. It had a nicely claustrophobic/surreal tone to it and the characters acted suitably desperate, but overall I just felt like the interesting bits (Namely, wtf IS Gantz, anyway?) weren't going to be explored by the author any time soon and in the meantime everything else was a slog.
*Admittedly, this is quite often an unfair generalization but even so it isn't going to be series like Gantz that changes the perception.