To be up front about it, I bought this because I'll buy pretty much anything with Neal Stephenson's name on it.
It is not, however, up to the levels of his other books. It's good, and worthwhile as far as it goes, but to me it feels kind of ... flat. It lacks something of a signature style, the stamp of one author, which is of course quite natural since it was a collaboration. It's not bad, it's an entertaining read, but you can definitely tell that it's a "book by committee". A very illustrious committee, certainly, but nonetheless, the book seems a bit "plain" compared to other things I've read by Stephenson: you probably couldn't pick him out as one of the authors from a sample of the book.
In terms of the subject material, it's about a band of warriors travelling to kill the Mongol Khan, and hopefully to stop their invasion of Europe. So definitely more of a historical fiction book than science fiction. Fine with me - I think Stephenson has actually done some of his best work as historical fiction. However, his other books all seem to add that trace element of the unknown that somehow makes them seem quite nearly science fiction books.
The only other thing that bugs me a bit, and a reason to put off purchasing the books, is that part III won't be out until early 2013 some time, so you're liable to forget some of what already happened by the time the book is released, if you read the earlier portions now. Perhaps it's only a sign of my aging, but it took me a bit to get back into Part II after a month between having first read Part I and getting the second one. Remembering all of the characters and what they were up to took a few chapters. I expect the same thing will happen with the final portion of the work.
Complaints aside, if you know and appreciate the authors in question, I'd recommend the book(s) - because it is fun and entertaining as long as your expectations are not too high. Not as one of their greatest, but as a fun read nonetheless.