Solid Game, I'm a long time fan of Zell as well and really appreciated seeing more of this genre. While it doesn't have the same combat / capture mechanics as FoBS or Succubus Affection, it has good gameplay and fun puzzles that change things up before things get too tedious. Pixel art is great and well made, I really appreciate the different dialogue depending on scenario in which the play is caught, and the variety of lines is refreshing as well. I'm incredibly biased since FoBS was my first H-Game really, but Drain Mansion really does its inspiration proud especially since it has definitive endings you can get. I also really appreciate that it has lots of fun secrets and encourages replays / hardcore runs. That all being said, there were a few things I would've liked to have seen that I didn't. There were definitely a good variety of girls, especially the horror themed ones, but I would've liked to see more, again especially the horror themed ones, as a fan of the erotic strange and bizarre, I would've liked more eldritch monstrosities in the cast. Still, there are some, and that warrants full points from me since most have 0. I would've also liked to have seen more art or had access to all the art hanging up on the walls all over the world in the gallery / close ups of them, cause some of them looked great. Also, maybe a little too much focus on the snuff stuff for the main hero imo, but it's pretty easily overlooked and doesn't get too crazy.
TL;DR Awesome game, solid gameplay, graphics, and writing, if you like Zell and are looking forward to Shiro no Yakata, give this a shot and you won't be disappointed!