Since Jim just wrote a very good and mostly spoiler free review of Under the Dome a week or so ago, I'm going to refer you to his review for the normal things like a plot summary. Most of this review is going to be taken up with my thoughts on the themes brought out in the story, there will be spoilers.
Under the Dome is, I believe, King's longest single novel, beating out The Stand by just a few pages, and it was solid. Fast-moving, exciting, even a little touching at times. If you've read King before, the things you love and hate about his writing are all here: heroic children, religious overtones, lots of folksy dialectical dialog, a large cast of idiosyncratic characters, a psychic child or two, and people getting killed in awful ways. It's a good book, but the most interesting things occur below the surface.
I SPOIL THE ENTIRE ENDING BELOW. HOVER OVER THE TEXT TO SEE IT.
After all human efforts fail to raise or destroy the dome, it's discovered that the force that created it is--drum roll--not of this world. That might not not be so strange on its face, since King has explored extraterrestrial life before, but in this case, the creatures wreaking the havoc don't appear to be extraterrestrial so much as supernatural. The antagonists--vague eyeless, leathery creatures--have more in common with Lovecraft's elder gods than with aliens or with the concept of an Old Testamental Abrahamic God, as King seems to indicate in his other novels. The creatures as described in terms of children massacring an anthill, incapable of feeling guilt or sorrow for what they do. Their ways are inexplicable and unexplained: the dome itself, in the end, appears to have been nothing more than some deistic prank, a giant lens placed over a town full of ants. It's interesting that King never attempts to answer the questions that would seem to be the thematic core of the work, except to say that there are no answers. Sometimes, the book leans toward God working in mysterious ways, but ultimately it goes a step further: the alien children, in this universe, are God, and nothing we do can induce guilt. We can only hope for pity.
As a result, I thought the story was a little bit of a downer. It's not really humanistic--nearly everyone in Chester's Mill is dead by the end of the book, and those that are saved are saved mostly by sheer luck, nor is it optimistic about any sort of supernatural intervention. The only ones who care about us are us, and there's nothing we can really do.