I sort of have the opposite problem with this book than the last Gibson I read. The last, Pattern Recognition, I liked the book quite a lot, but thought that Gibson didn't quite stick the landing. In this book, I thought the ending was fine, but it took too damn long with too damn little happening along the way.
The central conceit is interesting, if not anywhere near fully explored. At some point in the future, after a mass die-off of humans, the remnant have stabilized and have access to nearly unlimited energy and goods. (It doesn't seem to be post-scarcity, though, as there are clear mentions of people not being able to afford things. It isn't fully laid out.) A few of them also have access to a "conduit" that connects them with some part of the past. Not their past, as the future of that past started to diverge as soon as they made contact. The "stub" is what they call the past-that-is-no-longer-their-past.
They can't go there directly, they can just communicate by computer, and no one really seems to quite know why it works. They can do it remotely, through Skype, essentially, or people from the past can invest a humanoid robot called a peripheral in the future, and those from the future can do the same with a much more rudimentary version.
The main character is a woman whose name I don't remember (that's never a great sign) from the past, who was hired to act as remote security at a party in the future, having been told it's just a game. That seems unnecessarily complicated, but okay.
While there, she sees a woman get murdered, which leaves her, of course, as the only witness. This leads to people hunting her and the people in the future who had hired her trying to protect her as they try to get her into a situation where she can ID the murdered.
There's also a giant floating raft, which feels a lot like Neal Stephenson's raft in Snow Crash. And some nods to veterans with PTSD/loss of body parts and how peripherals would seem like the Holy Grail, but it's more hinted at than delved into fully.
The big problem comes with about a third or more of the book to go, when the police officer in the future says that what needs to be done is to take a peripheral housing the woman from the past to a party so she can ID the murderer. That's a good idea. Everyone agrees.
....and then more than a third of a book goes by where nothing at all happens. They talk about sending her to the party. An attack in the past happens, but in pretty much exactly the same way as we've already seen once or twice, The woman in the past gets shown around future London. BUT NOTHING ACTUALLY HAPPENS. For this honking great swath in the middle of the freaking book.
I mean, sure, some guy in the future pines for the woman in the past, but there's absolutely no chemistry there, and nothing ever happens with it. It's just a lot of...nothing. And nothing keeps on happening for pages and pages and pages.
Finally, we get to the goddamn party and things start happening again, and the ending is fine. It's that long padded pause in the middle of the book that's the problem.