It's hard to review short stories, because to say almost anything about them will give too much away, but I'll do my best to give appropriate commentary.
This was, for the most part, great, and at times brilliant. King seems to be one of the only authors today, especially in his genre, who consistently releases short story collections, though not all of the tales in Just After Sunset are horror. King is just as masterful at the short-form stories as he is at the monstrous novels, and that's more impressive than a lot of people realize. The short story is a craft all its own, and not an easy thing to pull off well.
Here are the stories that stood out most to me:
Willa and The Things They Left Behind are both pleasant for their easy pace and sweet quality. They deal with impossible things, as so many of King's stories do, but at the center are characters experiencing something scary that turns wonderful. While his imagination is wild, King can blend fantastic, other-worldly elements with regular people going through regular things and make it all seem perfectly believable. These two are like that.
Two stories have to do with exercise, at least they begin that way. The Gingerbread Girl is a smart, suspenseful little story of a woman in trouble. It is scary because what happens to her is completely possible. Stationary Bike is a curious story of warning, and no matter what you think the title means or where the story might go, you probably won't guess it; it's too out there. This story in particular had a concept so creative, I kept thinking about it days after finishing the story.
Mute is, ironically, a story told mostly through dialogue. It unravels somewhat like a mystery, though it is not. The end is chilling and rather unexpected, though not entirely unpleasant, or unjustified.
The Cat from Hell is an 'old' story, written (I believe) in the 1980's and anthologized a number of times, though it had never appeared in a collection of King's until now. This is a 'bonus-track' of a story, as King calls it in the Author's Note, and is a quick little nugget of absurdity, fun, and gross-out. With a title like that, you can probably expect what to receive, but it's a good story all the same.
A Very Tight Place also ventures into gross-out territory, though in a much different context. The title is fitting, for the situation in which the character finds himself is one of those that keeps you holding your breath as you rush through the pages, longing for him to get out of it. Again, this one is scary because it could really happen (though for reasons I won't say; that would spoil it), and is the embodiment of a fear that probably many of us have felt. This is one of the longer stories in the collection, and while it wasn't my favorite, I still found myself unable to put it down until I knew what happened.
N., the most recent of the stories at the time of publication and first released for this collection, is pure brilliance, and a flawless example of what a tightly-crafted short story ought to look like. The premise is big enough that it could have been a full-length novel, yet King is able to express the vastness of the situation in sixty pages or so. It has a fantastic slow-build, gently revealing the terrifying truths at the story's center, and generally has a foreboding heaviness about it; a cloudy darkness between the lines. Without saying too much, it has to do with obsessive compulsive behavior, and ancient, angry gods in another universe only a thin sheet away from our own. It is a story that leaves one feeling haunted, and viewing the world differently after finishing it, and that's powerful. I couldn't stop thinking about it. Also, this story casts a shadow upon the world of Under the Dome (that's not a spoiler, merely another one of King's many 'easter eggs' within his own fictional universe).
Those were the finest stories per my preferences, although all of the pieces in the collection are good, and have their own merits. This is a great little collection, and could easily be read straight through, or popcorn the stories one by one at random; either way, you'll have fun, and a few scares.