That convention is most relevent in the "alternate history" sub-genre. As the "hardest" of the first-wave cyberpunks, an SF fan has to expect that Gibson and Sterling would honor that core convention. So the greatest mystery of this book, for most of its length, is to figure out what the devil that one change _is_.
Since I believe I've done that -- and it's by no means obvious -- I won't spoil the fun. But I will say that it looks like much better SF once you do figure that out.
The book has many flaws, most traceable to the dual-authorship. The writing is uneven -- neither Sterling nor Gibson are chameleons, and they don't do much here to approach a common style. Characterization is uneven because, though it's a strong suit for both writers, they handle it quite differently, and seem to have different visions of the characters.
But even at its worst, this is a good novel; and it's one of the most finely realized and plausible alternative histories I've ever read.
I would love to read more works in this genre. Recently, there has been a renewal of interest surrounding the accomplishments of the Victorian era, and we should all keep in mind the spirit of possibility emodied by the Victorians. This is a good book to read in conjunction with Neal Stephenson's _The Diamond Age_, which I will also review.
And why do I especially like this book? It's the first instance I remember of what soon came to be known as "steam-punk." A technology-based yarn, but with Victorian techno, not computers. Not exactly. There are three principal characters here: Sybil Gerard, daughter of Walter Gerard, the great Luddite agitator and orator; Dr. Edward Mallory, dinosaur-hunter, afficionado of steam-gurneys, and stalwart of the Industrial Radical Party; and Laurence Oliphant, who pretends to be only a somewhat adventuresome journalist-cum-diplomat but who is actually a top intelligence operative and handler for Her Majesty's government. And then there's Inspector Fraser, part of the very Special Branch, as well as a number of nicely realized supporting characters.
But, of course, the story is really about the world of 1855 in which Charles Babbage was very successful in developing his mechanical computer, a marvelous Engine (always capitalized here) of wheels and rods and gears and punch cards that has put Britain well on top of things, and the government in many ways well on top of its citizens. The plot device that gets things going is the theft of a box of punched Engine cards, the purpose of which is never quite divulged -- though we know the program they contain is Important. It's all a great deal of fun in the Idea-as-Hero tradition. Gibson and Sterling (mostly the latter, I think) have definitely got the feel of the times and the city of London, immersing the reader in authentic jargon and cant, but without casting you adrift.
Frankly, I don't understand the antipathy of the other reviewers. It's a pretty good story and most people I know personally, even those who aren't big Gibson fans, liked it. In fact, my only real complaint is in an area where Gibson's hand definitely shows, and that's the ending of the book. Actually, it doesn't really end; it just stops, and with a bit of Gibsonian mysticism regarding the set of cards thrown in.