As I said, I tend to read new things as I see them. I read this a few months ago, so you'll pardon me if my review is a bit sketchy. I personally find the Russian Campaign to be one of the more fascinating events of the Napoleonic Era.
The author of Moscow, 1812 has some obvious anti-Napoleon bias, though you could probably guess that going into the book due to the subject and the subtitle of "Napoleon's Fatal March." For the most part he does keep this under control and tries to present the events fairly. While he does describe Napoleon in a very bad light politically he does give him credit for a decently fought battle at Borodino and attempting to do the best he could with some very bad circumstances most of the time... despite the fact that he had gotten himself into them.
There are some times in the earlier parts of the book when he calls Napoleon evil or a tyrant outright, which I find a distasteful historical practice when it comes to most historical figures as it is better to let the events, results, and intentions speak for themselves and allow people to make up their own minds from that. If you present a good case, you don't need such adjectives to make your own point clear.
Roughly the first half of the book sets up the events politically and with the major players. The political explanation of the reasons for the Russian Campaign is well done and lengthy and manages to avoid being dry even to someone who has read a few books on the subject. The second, and by far more interesting, half of the book deals mostly with the events in Russia and the retreat. This is told from a more intimate viewpoint with excerpts from diaries, letters, etc. This does get graphic. Very graphic.
Overall a very good book and worth reading if you can overlook some of the bias and you don't mind some very graphic content, such as in depth descriptions of the effects of frost bite. Mr. Zamoyski is also a very good writer with a touch of the old-fashioned narrative historian so it is simply enjoyable to read on that level.