I have been going back to books I read when I was young, to see how they hold up now. In truth this book left an impression on me. At the time the final resolution blew my little my little mind. The unicorns in particular captured my imagination, alien and intelligent in a way reminiscent of Pern's dragons (which I would read a year or two after this).
Having read it as an adult, I think the only reason I remembered this book is because I had yet to read many others like it.
In short: it's mid. I've read better, I've read worse. If you have this book somehow and some time to kill, go for it.
Now for the long of it. The thing that drives me nuts about this book, and what keeps it from being a truly good book, is the author's choice to write the story largely from the POV of one of the least influential characters in the plot. There are a number of times where other characters are telling him through dialogue about other, much more interesting events that happened to them. What the hell, I would've rather read that! An entire seige happens off page while the "story" lingers on several days of uneventful boat travel. Williamson should've cut most of Zorn's pagetime, and committed to the multiPOV bit more thoroughly.
It's really a shame because when he's not describing one guy's worst cross country roadtrip, his writing is pretty interesting. The scene from the perspective of the invading demon was legitimately compelling. Something that I had recalled about Demon Moon was the genre twist. The author does a pretty good job holding his cards to the chest, building that bridge between fantasy and scifi. I think that part holds up fairly well. That ending though was too neat and pretty haha.