First of all, I don't really think that VALIS is a science fiction novel, although I'm not really sure what VALIS is.
Phillip K. Dick apparently had some sort of mystical experience in 1974, he also did a lot of drugs and suffered from mental illness. VALIS is an autobiographical account of all that. It has two main characters Dick and his alter ego Horselover Fat. It is sort of a metaphysical detective story into the nature of God without any answer.
Probably the best way to think of the book is as the lunatic ramblings of a genius. It's very hard to tell what is real and what is not. The shifting perspective of the narrative is brilliant. I've read two Dick novel's prior to this one and this is by far the best one. It's full of interesting ideas, which Dick bounces between, which is how he writes. It's also about a crazy person - explicitly so, and I think Dick must actually have been crazy when he wrote it.
The best thing about it is the blurriness of the lines between the first and third person narration and between Fat and Dick. That is truly remarkable and if the whole book was that good it would be a five star book.
Untangling it all is too much for me, but the book is well worth reading and is maybe the ultimate mad-genius novel.