I think River of Stars is criminally underrated if we're talking Kay's output in general. The Sarantine Mosaic remains one of my favorite pieces of genre fiction maybe ever. The settings, the writing, the characterization is vintage Kay, who is one of my favorite authors of any genre of fiction.
I thoroughly enjoyed this latest book, I'd put it up as a top 5 book of the year. For some reason I stopped reading it because the Great Ordeal arrived in the middle of my reading this book, but I wouldn't have put it down for any other reason.
I think it odd he won his World Fantasy Award for Ysabel, which I really enjoyed, but compared with his other output isn't his best.