Like any Kubrick film, it's always interesting, occasionally fascinating, brilliant here and there. I saw it when it first came out, didn't like it much, and saw it again recently after a gap of 20 years. Surprisingly, my feelings about it haven't changed significantly. Nicholson is one of the best actors America has, but I have to agree with other reviewers I've read who have said it was a mistake to have Nicholson's character start off too-tightly wrapped, and then get progressively crazier as the film progresses. Yes, King's protagonist in the book had psychic burdens (alcoholism, an abusive childhood, anger issues, etc.), but where the book was effective (and it was often effective, although I thought it had too many ineffective parts and fell apart at the end) was in showing how an essentially normal, average man (after all, how many of us have anger and/or other issues?) becomes a monster as he meets whatever it is that possesses the Overlook Hotel. Because of Kubrick's approach to the subject matter, this contrast is never demonstrated.
There was a TV mini-series that came out a few years ago, sanctioned by King himself, that actually came closer to the essence of the book, although it had none of Kubrick's style of cinematic genius in it. I'm not a big fan of either film (or the book itself, for that matter), but it rings a lot truer to what King was trying to impart, if you're a King fan, than the Nicholson film.