I read all his novels in my 20s and loved them. I tried reading The Devil Rides Out again a few years ago, but found it so dated I quickly abandoned it. I think the novels are very much of their time. (The scariest, probably due to my claustrophobia, was The Ka of Gifford Hillary.)
I was very interested in the occult back then and I still have his illustrated, The Devil and All His Works, which is quite fascinating.
In the book he talked about the evil lurking under the pyramids at the abandoned ancient Mesoamerican city of Teotihuacán near Mexico City, and when I visited there some years later I definitely felt a very odd and unsettling 'presence' there and quickly exited one of the underground passages in a cold sweat. I've often wondered if I would have felt anything if I hadn't read Wheatley's book, but whatever caused it, it was still a pretty wild and memorable experience!