Plenty has been written here already about the storyline, and anyway in my view less is more when it comes to knowing the plot in advance.
What I look for in these reviews, and what I attempt to give back, is some clue as to whether I personally will enjoy the book. In this approach I end up saying why you might not like it. A reverse recommendation if you will. An inversion.
First off, Inversions is not a classic Culture novel. By classic, I am thinking of the novels of scale. A Player of Games springs to mind. It deals with the Culture on a macro level. We are privy to the bigger picture as the story is recounted. In fact, the storyline is merely a device to introducing to us the nature of the Culture as a whole. Storyline as tour guide.
Inversions does it differently. It deals with a subset. A story within a story, a personal account of what happened. We are not given the bigger picture, there is no macro level narrative. We have to fill in the blanks for ourselves. Such a story can only make complete sense if you know the Culture already.
The story does not fail if you are not Culture-wise, but without that wider understanding your view is blinkered.
Secondly, as mentioned above, this book is a personal account. Rather, it is two personal accounts. The focus is on the people, on the characters - this is pretty much an obvious consequence of such a narrow focus. It is a book about people not things.
As an aside I heard someone on the radio suggest that women like people and men like things. A bit generalised, but enough truth in it to be worth remembering. Inversions is a more feminine book.
So my second 'warning' is that you are not going to revel in GSV's, Superlifters and Plates. Even less Minds, CAM and tightbeam transmissions. Im pretty sure most fans of Iain M Banks will not be put off by that, but some will. On a certain level it could be said that this is not even sci-fi, although I would contest that point.
There is a danger that I am making Inversions sound like a huge departure for Banks. It is not. But it is a little different. Some people will think this is his best novel yet, and others his worst... and hopefully I have provided some clues to which camp you might be in.