Ragged it, then. It was good; as pacy and imaginative as the other two Culture novels I've read, Consider Phlebas and Excession.
I think the problem I had is that when I read the synopsis a couple of years ago, I jumped to the conclusion that with the theme of Subliming, and it being published only a year or so after Banks died, it would be a sombre meditation on the transience of existence and the nature of the afterlife. And of course though there are touches of that, its really just a big James Bond In Outer Space, with characters traversing the cosmos to all kinds of exotic locations before an action-backed finale in a huge what-might-as-well-be-a secret base.
Twenty nine novels in as many years makes Banks a highly prolific author (especially given the size of some of those works) and I did get the impression that a lot of The Hydrogen Sonata was written pretty frenetically. Still, plenty to like, such as unfolding secrets of the Zihdren, the Liseiden/Ronte sub-plot; and of course, those garrulous Culture Minds.