I'm going to be a voice of dissent. I don't think the fluff in Rifts is that great.
IMO, the first Rifts book presents a pretty interesting post-Apocalyptic environment, but subsequent books make the setting worse and worse.
1. There are far too many unimaginably powerful tentacled alien intelligences swarming around Rifts earth for my taste.
2. Human villains in Rifts tend to be one-dimensional cartoons, described as psychopaths or sociopaths and given no further motivation.
3. There are tons and tons of very silly giant robots and suits of power armor in the supplements. Not only are the designs silly, the more giant robots there are, the harder it is to believe in a wild, untamed, dangerous setting.
4. Siembieda's world building tends to be ad hoc, and it focuses on stereotype and cliche. IMO, the pieces don't fit together very well.
CAVEAT: I walked away from Rifts ten years ago and haven't looked back. It might be super-awesome now.