For those in broad agreement with his philosophy, it was seen as groundbreaking. And to be somewhat more pedestrian, people were fascinated by the concept of daemons (because they're a reflection of the self, and people are intensely interested in themselves).

I liked them because they were innovative. Fantasy is a bloated genre and there's a lot of crap to wade through, but some of the ideas in HDM were genuinely new. For that alone it was worth reading.