I think Chickamauga, and the pre battles before it, is where the game begins. The battles up to Cold Harbor and afterwards are by far my favorite and the ones I've spent the most time on.
The key to this game is preparing your army specifically for these battles. Granted they are extremely difficult especially on the 2 hardest levels but that is what makes this game so fun. It would suck if you could just walk through them. If calvary were a little bit more manageable this would be the perfect civil war game.