The system in general I think really captures the period - lots of 'flavour'. Plus, I have grown to very much appreciate the late Rick Barber's artwork, which again, adds much to the period 'feel'.
Also, I've always thought that its interesting to play ACW where the units represent regiments (organised and activated as brigades) and batteries.
The whole thing really, not forgetting aspects such as the command system, the random events, the way cavalry is depicted and the wonderful combat system. I really have nothing negative to say about it, other than they are, for me, anyway, a bit expensive. I was looking at the South Mountain game, but with shipping, even from a 'local' retailer, it would be almost £100 ($122) which is crazy for a game that, in the US, would cost $85( +shipping extra).
“O Lord God, let me not be disgraced in my old days.”
'We few, we happy few, we band of brothers'