This is going to feel more like a review of the collection itself, rather than of the individual stories. Why? Because I already really enjoy Michael Shea. He's capable of really humanizing more esoteric horror and fantasy by grounding it in believable, identifiable, and realistic characters. He can do action that feels like a more believable Howard. At least, when he's at his best. And the initial half to two-thirds of this collection is some of his best.
Demiurge is as close to a complete collection of every cthulhu mythos related tale that Michael Shea wrote as you're likely to get (there's an outlandishly expensive Centipede Press collection that's probably more complete). I do not necessarily know if that is a good thing.
You see, its essentially the Perilous Press edition of Copping Squid, with a few other stories tacked on to the end. And I mean tacked on. Its clear to see why several of these entries may have been omitted from that collection, as they just don't hold up by comparison. Some of them don't even feel related to the cthulhu mythos besides stylistic similarities to Lovecraft. Taken on their individual merits, there are a couple of fun additional stories though.
Momma Durtt was fantastic, creative, original story about gangsters, truckers, poor small town folk, and maybe some sort of cthonic entity (or something else entirely?). For me, it had echoes of Ambrose Bierce's "Damned Thing."
Under the Shelf fooled me. Its opening with characters preparing to explore the ocean below the ice shelf had me prepped for a Jules Verne, Jacque Cousteau, Abyss, but-with-some-more-horror style adventure. It quickly and surprisingly morphs into one of the most action packed (and to some degree sci-fi channel movie goofy/campy) Shea short stories I've read.
Ultimately, though, none of the individual stories added on to this collection are good enough that you really need to read this in addition to Copping Squid. Barring being a completionist. ( )