Are we still talking about The Years of Rice and Salt? I actually read the book.
The history aspect is absolutely ridiculous and suffers from the same problem a lot of other alt-history books seem to have, in that they merge the entire world into just a few single entities. The idea of a World War One not only breaking out in the timeline, but it being a three-way fight between China, "all of Islam", and an Indian-Native American-Japanese pro freedom confederation is just so dumb, its awful. Don't go into it expecting good history, or anything of the like.
That being said, the book is fantastic for the reason I think Dach's' was hitting on early, in that it's a very entertaining read. Kim Stanley Robinson knows how to tell some very interesting tales, and mess with the various settings he gives himself. Most of the book doesn't deal with the political history behind whatever weird crap he's coming up with, but rather stories about select people during each of these time periods. The overarching narrative about reincarnation in the book is also very well done, and helps weave the various stories he brings together.
Overall I recommend it, though not for the reasons you seemingly wanted to get into it for. It's a very good story book, nothing to do with any actual history or look at serious alternate history.