"Reploids" was the only Stephen King story I hadn't seen in other collections. The other two, "Sneakers" and "Dedication" can be found in "Nightmares and Dreamscapes" (1993), which I fully recommend buying instead of "Dark Visions" if you're a Stephen King fan. Because "Reploids" is a just a blip, a short story never reprinted anywhere because it's not really all that significant. ONLY worth it if you aim to read ALL King's works, no matter how small or obscure.
Dan Simmons' "Metastasis" just plain sucks. SO unworthy to follow King in any book.
However, the other two Dan Simmons stories are good. Not gimmicky like the first one, but really, really very nice. "Vanna Fucci Is Alive and Well and Living in Hell" is absolutely worthy to be in any collection with Stephen King in it, because it contains some very well-aimed criticism of televised religion business, and also some nice existencial horror mixed with some plain good gross-out. And "Iverson's Pits" just might make you see war, and dying in it, in a different way.
All stories date from 1988, just like this collection itself, actually. So I expect to find GRRM's story in some of his other, later collection.
So yeah, if you expect to find something new here, you won't. Except for Dan Simmons, I guess.
Also, this book was very hard to get, even from Amazon, I had to wait for 3 months until they finally got it in stock.