(Minor spoilers) Although this carries out a number of SK favorite themes (other worlds than these) and images (low men with voices that sound like their voice boxes are filled with insects), it reads like a story that would have been published in a science fiction magazine rather than a Stephen King book. It's not a horror story so much as a short character study with interesting concepts. Maybe it's the result of making a shorter novella from some of the fabric of the Dark Tower books and fitting it onto a (virtual and literal) Kindle. Maybe it's because it has a different kind of ending than we've grown accustomed to. But it's a nice, tidy, self-contained story that doesn't go too deep and stays within the world of ordinary nice characters that could have inhabited any earth-bound F&SF cover story for the past 30 years. It was a good read.
The most interesting things (especially since I read it a week before Election Day 2016) were the alternate US histories, both banal and terrifying, that were revealed. (Not even SK's most outrageous UR dreamed up in 2009 could include the possibility of Donald Trump as president, though... I think we got shunted onto a little-used level of the Tower when Al Gore lost the election in 2000).
My lack of the fifth star was mostly because the low men weren't consistent with the rest of the DT universe. These were unpleasant but not terrifying guys, and they actually seemed likable compared to Ted Brautigan's pursuers. And they wore the Red Eye but said "all things serve the Tower." They were Regulators, maybe, but not the terrifying type. They weren't bent on the destruction of the multiverse, but on maintaining the status quo and not working too hard at it. So maybe they should have been frumpy guys in cheap suits who looked like IRS agents. In my humble opinion.