Ooh, I love me some concrete yet inexplicable manifestations of God in the real world. Loved it, in general.
At the beginning of the story I was suffering from quite a bit of conclusion about the proportions and location of the God-hand. At first, I thought it was hand-size and hovering over just this kid's head. Then I thought it was town sized but suspended in the air like a cloud. Then I realized its cupping the town protectively, closing off ground access on all sides. It took me way too long to realize that, and along the same lines I didn't realize that they were all growing shrooms for food because the hand was blotting out the sun.
I really dug it in general, with the shrooms and the hand and whatnot, and him trying to escape. Then at the end, I had the same reaction some others had where it suddenly felt like a completely different story when the outside world was revealed and they were given a similar drive.
"First one's free. After that you run away screaming and never come back because oh my god what IS that THING!?!?"
Yeah, my one problem I had with the story was that. Generally the "first one's free" business model works if it's something that you feel you can't do without. If there weren't the mutants out there, being able to bask in sunlight would certainly be such a draw, both for psychological reasons, and for vitamin D deficiency. But with the mutants, it's a pretty terrible model.
To save my liking of the story, I'm going to go with an interpretation that I suspect was not intended by the author, that the shrooms are making everyone hallucinate, but that they're a psychically shared hallucination so that they're all seeing the same thing and interpreting it the same way. Yeah, I don't that was intended, but then I can be more happy with how the story went if I pretend that's what was intended.