So in the first... half... or so of Gantz, the one thing that comes to mind when is "Holy cow, the protagonist is a pervert!" And he is... and not in the comedic played-for-laughs way, either. And could the girl he likes be any more useless? Masaru is really the only decent person in this bit.
However, lots of Character Development happens around, say,chapter 140 or so, and Kurono graduates from being a total butthead to... maybe about 20% butthead. You really begin enjoying the story around this point. Before, it was just an 'okay' manga, with great action and a shallowness rating of 10, but around here, the protagonists kick butt and take names, and it's amazing.
Unfortunately, it later devolves into this really strange alien-invasion story (not like it wasn't one in the first place, so I don't feel like I'm really spoiling anything), and some of the fights are truly interminable. The sheer scale of it makes it hard to tell what's going on. Toward the end of about 300 chapters, you start wishing it would be over already.
To sum it up, in the first 100 chapters, the main characters mostly think with their hormones. In the second, it's a great story. And in the last 100, you start skipping chapters.