A breakdown for all of you:
Follows the manga plot, except Kashiwa plays an extremely minimal role. She doesn't even have a name. Satou did, however, bang her. Satou doesn't go to a maid cafe or anything, but does re-enact Fight Club with Yamazaki and sneak into Misaki's church to watch her be a total spaz. There is no online gaming or pyramid scheme shit at all. Yamazaki completes his game before leaving, but it is a suck-ass F-movie grade Dragon Warrior knockoff. Yamazaki is also fucking dark and crazy in this, seriously.
Anime: QUALITY Gonzo animation, but follows the novel closely and includes all the good stuff from the manga, minus the strange Fight Club narrative. No drug references at all.
Manga: Totally spins out in a different direction. Still good if derivative; really pushes the boundaries of believability, loses itself around volume 6.