I'm not sure that I'd self-identify as a Stephen King "fan", because I've only read around half of the books he's written and I've never really felt the kind of intense connection/devotion/desire-to-read-them-all that I do with books by authors who I am a fan of (Donald E. Westlake, Philip Roth, Ross MacDonald). But I am a Stephen King defender: I'm a big booster of three of his novels - The Shining, Pet Semetary, and The Dark Half - and I've (more-or-less) enjoyed most of the other ones that I've read (especially The Stand, Bag of Bones, It, Dreamcatcher, and From a Buick 8). While it's not an argument I'd want to get into with people who think like Harold Bloom, I do think that King is one of America's greatest, contemporary pop novelists.
But if Insomnia was the only one of his books I had ever read, I'd probably agree with his critics. It isn't a terrible book as far as these things go: it's an easy read if only intermittently compelling. It did have one moment that genuinely freaked me out. For the most part though it was disappointing. However, it's flaws are characteristic of the problems that even King's best books have.
King is really good when he's writing about creative guys who are more-or-less his own age from more-or-less his own background (see Jack Torrance in The Shining, Thad Beaumont in The Dark Half, Paul Sheldon in Misery). He's, um, less good at writing characters that are different from him - working man characters (Paul Edgecombe in The Green Mile), African-Americans (John Coffey in The Green Mile), or, in the case of Insonmia, senior citizens. Specifically, his problem seems to be that while he keeps a critical distance from characters that are "Stephen King-like" which allows him to bring real insight to bear on them, he tends to get sentimental and fuzzy about characters that are different from him.
If this was the only problem with the book, I might be willing to overlook it. For instance, his Hard Case Crime novel The Colorado Kid is unfortunately marred by his dopey and sentimental characterization of some way down east Maine old-timers. But The Colorado Kid is otherwise a very nice meditation on mystery novels and it is very short. Insomnia is long and, even apart from the sentimentality, most of it is just "bleh".
King is such a prolific writer - both in terms of the number of books he writes and the size of each book - that it's not surprising that he isn't always "on", but it's when you're halfway through something like Insomnia that you wish he didn't feel he needed to publish everything he writes. Especially, since lots of Insomnia read like a warm-up for the (much better, not to mention more consistently frightening) Dreamcatcher.
Still, even some of his best books have too many pages. Dreamcatcher would probably have been helped if it had been 100 to 200 pages shorter than it was.
But King's biggest weakness may be that his set-ups always tend to be a lot more interesting, exciting, and engaging than his endings - which often feel too similar to each other: the action wrapping up with some vague supernatural, super-powered, mumbo-jumbo-flavored battle.
Hey - slogging through the final scenes of Insomnia I was reminded that I had a very similar problem with Neil Gaiman's American Gods and then an epiphany: as you take the overtly scary stuff out of King's books they start to look an awful lot like a Neil Gaiman story. So much so that a book like Insomnia - which really doesn't have many scary moments at all - might as well be labeled "fantasy" instead of "horror". Anyway, both King and Gaiman have an annoying habit of reverting to a lot of hand-waving whenever they start to deal with magic/psychic/supernatural effects. I'm not suggesting that their books need a fully-worked out "magic system", but I think they would benefit from their authors writing about magic with a bit more specificity and a bit less of the philosophy that magic just happens to do whatever is most convenient for the story at that point.