Great author, no. Marvelously entertaining, and, yes, a great storyteller. (I appreciate the distinction, @finkelitis.) He has also written some total duds. Look in his short story collections if you want to see the spectrum. He obviously terrifically enjoys doing what he does, and I admire that. And he has a spectacular imagination, richly infused with myth and folklore and Jungian symbolism. Neverwhere is practically textbook Jung. But he is writing popular literature, not the stuff of great novels.
A couple of years ago I heard him give a reading at a nearby university. He read several pieces, including a chapter from what is now The Graveyard Book; he said he thought it might be chapter 4 of a yet unwritten novel, and he was just trying it out. (The audience gave it a big thumbs up.) When asked what came before and after that portion, he said he had no idea yet.
He also said that he and Terry Pratchett occasionally ask one another, “What do you suppose Aziraphale and Crowley are up to these days?” and agree that maybe one of these days they should put their heads together again and find out.