I believe there are two kinds of Iain Banks readers: those who are in it for the plot, and those who are looking for spectacle. Banks does both tremendously well, but hardly ever in the same book, which means that invariably reviews are split between people who thought his most recent novel was amazing, or merely very good.
I tend towards plot, myself. I think Banks is at his best when he keeps the scale small, and finds ways to twist and undermine his setting of high-tech, post-scarcity, socialist space dwellers, the Culture. Nobody does huge, mind-boggling scenes like him, but at those galaxy-spanning scales (and when starring the near-omniscient AIs that run the Culture) it's hard to feel like there's much at stake. My favorites, like Matter or Player of Games, combine the large and the small convincingly, hanging the outcome of huge events on the shoulders of fallible, comprehensible characters.
But for his last two books, Banks has tended more towards the huge-explosions-in-strange-places side of things. 2010's Surface Detail spun up a war in virtual Hells that spilled into reality, and now (with The Hydrogen Sonata), he's taken a look at a civilization trying to reach closure, even while long-kept secrets keep pushing up into the light.
I re-read Surface Detail this week, and I like it a bit more than I did the first time around. I still think it suffers from a lack of agency surrounding too many of its characters, who end up simply as pawns being ferried around to each major plot point, but I'll admit that those characters are charming, and the idea of the Hells--virtual worlds set up to punish people even after religion is technically obsolete--is thornier than it first appears.
The Hydrogen Sonata has a lot of the same issues: the events of its plot, while fascinating, are ultimately of dubious importance, and it's not entirely clear if any of the characters actually have real influence on anything that happens. But to its credit, the events of THS are so diverting, you almost don't care. This is Iain Banks doing spectacle at a level he hasn't really tried since Excession, and to a surpising degree it works. It's widescreen science fiction, and he's clearly having fun writing it.
The book opens as the Gzilt, one of the original co-founders (but not members) of the Culture, have decided to leave the material plane and "sublime" to a higher order of existence. Just as they're counting down, however, representatives of another sublimed civilization contact a Gzilt ship, hinting that they may have planted the seeds of Gzilt religion eons ago (and thus prevented them from joining the Culture when they had the chance). This sets off turmoil in the local government, and a gang of Culture ships recruits one former Gzilt military officer, named Vyr Cossont, to hunt down the oldest living survivor of the civilization's founding for a first-hand account of events.
There's not much actual mystery to be had here--Banks telegraphs how things are going to end up pretty quickly. But the fun is in the oversized set pieces being tossed around one after another, from the "Girdlecity" (a giant, elevated metropolis wrapped all the way around a planet's equator) to the hapless group of insects who conduct bee-like dances with their spacecraft while waiting to scavenge on the remains of the sublimed worlds. There's a Last Party being thrown by one rich Gzilt before the subliming that continually tops itself in extravagance. I was also tickled by Cossont's quest to play the titular composition on an instrument called the "Antagonistic Undecagonstring," which means she ends up lugging a bulky and inconvenient music case around the galaxy despite herself (as a bassist, I sympathize).
But while it's enjoyable enough, playing with these toys that Banks assembles, it's hard to shake the feeling that it's all a bit lightweight. The Culture has been set up in these books as tremendously powerful, almost omnipotent--it's run, if that could be said of decentralized anarchosocialists, by AI Minds at the helm of massive, powerful starships, far outclassing any of the other civilizations in the book. When there's a question of how events will turn out, it often reduces to "can ship X reach destination Y in an amount of time defined by the author?" which is not very dramatically satisfying. Like Excession, my least favorite Culture book, much of The Hydrogen Sonata takes place in catty infodumps between the Minds--these can be funny, but they can also read like you've wandered into someone else's e-mail thread by mistake.
Still, for people who are die-hard Culture fans like me, we'll take what we can get--even if I'd rather see more plot and less spectacle. Books like The Hydrogen Sonata flesh out a rich, funny, dark universe that Banks has been building for 25 (!) years now. It's good to visit, if only to point and enjoy the sights.