I have played the demo, and although it was similar enough to the previous game to jump straight in, I really like some of the changes. For example, being able to "sidestep" line formations makes setting up positions so much easier.
As for replay value, I can only offer my own opinion and that is multiplayer will give it immense longevity. I can only begin to imagine the possible matches, scenarios, mods etc that could keep it going for ages. I played Sid Meiers Gettysburg (not by the same people as this game for anyone unfamiliar with the games, but very similar) in co-op mode with two friends playing the entire first day of the battle. It was truly awesome, with moments of excitement, chaos, desperation, blame and pretty much everything else thrown in
As soon as I can spare the money, I think I'll be buying this. Will probably be playing it with the same friends i'm still playing Ageods ACW with, but will also look forward to some of the big mp games mentioned!