I really like this book. It might not make my top ten (there're so many!), but definitely my top twenty. I like the science fiction element, and I like his small-town architecture. I think the concept is interesting, if not all-together original (see BBC serial Quatermass and the Pit).
Small-town building is one of King's greatest strengths, and it's on show front-and-center in this book. Yes, he's done it better, with Salem's Lot and It and others, but it's still executed well here, absorbing me into the rural minutia and the day-to-day lives of minor characters.
I love reveling in that voyeuristic opportunity he offers of the town, much like the milkman chapter of Salem's, and the pink-grapefruit scenes with Aunt Cordelia of Wizard in Glass, not to mention many more. (I believe Salem's Lot's town dynamic is what inspired Peter Straub to craft his masterpiece Ghost Story.)
King loves to build up these living, breathing microcosms of Middle America (Derry, Haven, Castle Rock, etc.), just to tear them down. And I think that's one of the best parts of books of this nature: witnessing the horrific and utter destruction of these towns by a true master.
By his own admission, King calls The Tommyknockers "an awful book".
I love it, for whatever that's worth.
On a side note: Anybody here on the board familiar with the poem growing up? King says Tabby also knew it, or a version of it, so he didn't make it up. I'm from a multi-generational New England family and none of my family had ever heard of the Tommyknockers before King published the book. Personally, I was three when it was released, so I have no reference to it but King.