The world of On is one endless wall to which its inhabitants cling, living small and precarious lives on crags and shelves, never more than a few feet away from the edge of the world.
One day, Tighe, a villager, falls off the wall and onto another ledge whose existence he's never suspected. There, an army is preparing for war, a war into which Tighe is pressganged. His fight takes him to forests where there are man-eating insects and into slaving cities and to mountains of ice and eventually he discovers (or, rather, the reader discovers) something about the nature of the wall itself.
Which all sounds quite fun. Unfortunately, it's not.
For a start, On is slow-moving, especially at the beginning, and what should be an average-sized book (385 pages) feels absolutely interminable. To make matters worse, none of what happens is very interesting: nothing in the book made me care about Tighe, about his losses and his gains. Also, Tighe is a fairly unpleasant character
The plot itself just feels like - well, it feels like the novel I'm writing for NaNoWriMo, if I'm honest: like Roberts is just making vaguely plausible stuff up to hit his daily word count. It's bizarre. None of it seems to have any connection to anything else that comes before or after it. And some of what happens is pretty horrible: the scene with the man-eating insects is not at all for the faint of heart.
Actually, thinking about Roberts' work (he's an SFF critic as well as a novelist) I suspect that On is actually an extended literary experiment: it's about change, flux, how nothing is still and nothing stands. Which, fine, but it doesn't make for very interesting novelling. There's nothing vital about Roberts' writing here, nothing urgent. It's simply detached, dry and really bloody long.