Let me explain a little, my friend threw this on my harddrive, I knew nothing about it, other than it had some noteriety. My other friend and I were hanging out and he wanted to watch some anime, we started it, and finished all of it that night and the next day, we couldn't stop watching. Also, note, this anime is for more mature audiences, has nudity and lots of blood/gore.
Gantz...is... amazing. Let me start with praise, this anime does things animes (and every other media) are afraid to do. Without giving away spoilers, it takes an interesting approach that other medians of media are afraid to do, Gantz will do what you aren't expected and chances are pretty high, it really happened!
The pacing in this anime was very unexpected, I figured it would follow the typical pattern of, set up a new world, go there once an episode, have a problem and wrap it up, episode over, hell no, not here, Gantz hits you with with suspense (which I think is why some of the younger audience won't like it, due to its slower pace).
Characters show real, get this, real development! You'll begin to care for your characters, and totally disregard others. The pacing for this development is expansive and never truly stops.
It looks great, period. The sound works with the suspense and at other moments you don't notice it (which typically means they are doing a good job considering you don't feel the music is off). The story is neat, but you're more invested for the characters and what will happen to them.
A MUST WATCH