I only ever read the first two books; my parents bought me a volume that contained them both, and I really loved them when I was little, reading them over and over. I wasn't aware that there were further books until much later, though. Thanks to Fenrix, though, I just downloaded a collection for my kindle.
As for the story itself - I thought it was really good. My one nitpick is that I figured out the nature of the decay long before the characters did - not its origin, but the fact that it's coming from Jack's body rather than the pumpkins. I mean, his body was the one thing constant during all the experimentation. I was rather frustrated as to how it never even occurred to either character until the ending - though, in retrospect, Jack was going mad and the narrator, whose name I can't recall, was a farmer, not a doctor. But then again, neither am I, and it still occurred to me.