Declining?
Declined. I stopped reading his predictable, formulaic crap over a decade ago. If you've read 3 Stephen King novels, you've read them all. And I did read them all for a while, every one - even the Richard Bachman stuff - and thought it was good. Until about 13 years ago when I realized, in fact, it all sucked. Either my tastes changed or Stephen King was never that good in the first place. I vote for the latter.
Not about the fact that King's talent-or at least, his ability to harness it-has decreased markedly in recent years, but with the assertion that his writing is predictable/formulaic.
Even novels that appear to be rehashes of old concepts, e.g. "From A Buick 8," are actually quite innovative and unique once you start to delve into them.
The quality of his work has declined perceptibly-I trace it back to his recovery from alcoholism and substance abuse problems, while others cite his nearly fatal accident on that nearly deserted Maine road-but that doesn't mean that he can't still produce an enthralling work, as the final three volumes of "The Dark Tower" demonstrate.
I personally prefer the early part of his career, especially the original Bachman Books, as well as some of his early short story collections and novellas, which stand up as his finest work to this day.
Unfortunately, he seems to have lost his edge, the bite that you saw in novels that dared to ask viscerally disturbing questions, such as Pet Sematary.
Go back and look at some of the plots for his early work, e.g. ritualistically abused adolescents turning their frustrations onto their classmates, individuals using passenger jets as cruise missiles, habitual smokers being targeted for sadistic persecution, eminent domain abuses escalating to the point where they engender unspeakable carnage, viral pandemics wiping out vast swaths of humanity, etc...
I'm not saying that these issues had never been broached-even in literary terms-before Stephen King, but he gave them a broader context within the horror genre, which had never occurred before.
Critics like Harold Bloom, among others, dismiss his work as mindless dreck because they see it as banal pulp fiction, when I think the truth is that his genre has allowed him to explore some of the most pivotal cultural and philosophical issues that other writers address in a more opaque manner.
That being said, I still think he's no longer in top form, but I will defend the DT to the death.
:-)