There is an interview at the back of this book in which Banks says he was thinking of giving up writing SF but he set himself the task of creating a completely new context for a novel; The Algebraist, Banks' best novel for years resulted.
With Matter Banks returns to the Culture - and that is a mistake. Every worthwhile idea relating to the Culture has been expounded multiple times already - there has been no need for a new Culture novel since Use of Weapons and the quality of them has been deteriorating ever since. (It seems likely that Use of Weapons will be the best book Banks writes in any genre, ever.) This means that when we are exposed to yet another rehearsal of the arguments for and against interventionist politics, it is just boring; Banks fans could present both sides of the argument without having to think by now. Some of the characters are also Banks cliches and all of the main characters spend considerable time merely travelling from one place to another before they can meet up for a climax that is too short and unfortunately predictable, at least in general outline.
Once again the book is too long for its own story; if ruthlessly pared down to half its length it might move fast enough not to lose the interest of its readers. This affliction is so widespread amongst contemporary authors that one must suspect that the publishers/editors must find it somehow desirable.
One aspect of the book, is superior to The Algebraist, at least - there is very little reliance on crude jokes to bulk up the story, which is about all that the middle part of the Algebraist consists of.
Is Banks a spent force? It seems the man who shook up hard SF and made it a powerful force again may have been overtaken by newcomers to the genre.