I've only read a few stuff, but I largely like him.
Edge of Dark Water is probably the best book I've read by him. An extremely dark and clever depression era thriller that has elements of Night of the Hunter (which, if you haven't seen, is an incredible film).
I liked the Drive Inn books OK, but they get kind of wonky as they go on. Still, I would have loved the movie that was never made by Don Coscarelli (who later did Bubba Ho-Tep)
I also read his Jonah Hex comics. The first one is great, the second one is, like Drive Inn's sequels, a bit wonky (though I love the Oscar Wilde aspect of it, especially the flashback) and the final one feels like an attempt to make a less crazy story, but is just OK. All of them are currently collected in the book Jonah Hex: Shadows West.
Oh, and I loved the episodes he wrote for Batman: The Animated Series (even Critters, though it is very silly, it really feels like the perfect combination of his and Steve Gerber's crazy, wacky, horrific writing styles.)