I read all of them about five years ago and loved them, but then I stopped and haven't read any of his books after Knees Up Mother Earth. They are not the most intellectually stimulating books, but they are the closest I can get to hallucinating through legal means.
I was just watching a documentary about Terry Pratchett, and I think I saw one of his fans getting him to sign a Robert Rankin novel.
I understand that there is a certain rivalry between Mr Pratchett and Mr Rankin but I have always enjoyed both their works especially now that we're not going to get any other Discworld Novels anymore
It's funny that you should mention "Knees up mother Earth" because I'm currently rereading that particular novel (and enjoying it greatly).
I agree that Rankin's books are not the most Intellectual of reads but then that's not why I read them. I like the Metafiction and the general otherworldly sense of the Novels especially in the now legendary Bentford Trilogy.
Not to mention the running gags...