I was a reasonably early convert to Iain M. Banks, picking up his first book, Consider Phlebas, in a bookshop, and falling headfirst into complex, sophisticated Culture. I have read every Culture novel (though not all the stories). I've enjoyed most of them, though as I noted in my review of Surface Detail, it's the society that's memorable more than the characters or the plot. That's true here as well, and I think it's fair to say that the plot of The Hydrogen Sonata is straight up weak.
That does't mean the book isn't enjoyable - it's got Banks's usual smooth writing, interesting moment, and understated humor. However, there isn't much innovative about it. I honestly doubt that a year from now, I'll remember much more about it than a few of the decorations - like the Undecagonstring instrument, which itself feels forced, and something of a token effort.
As noted, the plot here seems something of an afterthought - the mystery we're waiting to solve is revealed fairly early, making the end somewhat purposeless. While much of the action concerns a society Subliming, we learn very little about the process (or about the Sublimed themselves), and again it's treated largely as an afterthought - "There's this vastly important, culture-central event that we must all focus on and prepare for, and ... oh it eventually happens."
While the writing is nicely done, I found the book overall slow going, hindered in particular by its seeming aimlessness - a concern borne out by the ending.
I wish I could say that Banks' last Culture novel was a fitting work of genius, but it's not. It's a very run of the mill meander, which is still a decent book by others' standards, but a disappointment here.