I managed to obtain tickets to a sneak peak showing of this movie, which I just returned from. Here's my thoughts:
For one thing, the movie is 3 hours and 40 minutes. However, it really doesn't seem like it; the time really goes quickly. The main focus of the movie is Stonewall Jackson, and Stephen Lang did a really excellent job of portraying such a complicated character. Jackson was an extremely devout, God-fearing man, yet he was utterly ferocious and merciless in battle, and a little insane. Robert Duvall did well as Lee, but actually had very little screen time. Jeff Daniels reprised his role as Joshua L. Chamberlain quite ably, and many of the actors from "Gettysburg" return in GaG. Bruce Boxleintner (Gen. Longstreet) and Mira Sorvino (Fanny Chamberlain) could have been much more developed characters, but the directored decided to focus much more on Jackson.
My main beef with the movie is that it didn't present the most balanced view of the war's causes. Slaves are shown, but they all belong to 'noble southrons' who treat them like members of the family. Much less time is spent on Northern characters. There are very heavy, drawn-out moments where Stonewall is praying to God. Some of these probably could have been edited out. Another thing that bugged me is the fact that the story goes from the end of 1861 then immediately skips to the Battle of Fredricksburg. The Seven Days', Second Bull Run, and Antietam were all ignored, which is annoying because these were important parts of not only the novel, but also the war.
Overall, I recommend anyone interested in history see this movie, but the majority of the moviegoing populace might want to skip it.