I suggested this game on Polsaker's Steam sale post without having actually played it. I did play Ultimate General Gettysburg, which I suppose was the sort of beta/demo to this game. It was pretty good but fairly short in comparison to Civil War, plus it had a cheap looking UI and some issues with unit balance/attributes.
Civil War is the best military RTS I've played outside of maybe Total War. It was actually made by a Total War modder, and some of the influence shows. The game does deal with the biggest problem that the Total War games had - the AI. AI in this game will fuck you up without remorse if you make even the slightest mistake. Flanking is difficult to pull off correctly and requires planning, you will find yourself outnumbered more often than not and the game has ditched the "sandbox" style of Total War and replaced it with a kind of sandbox-trigger event hybrid.
The American Civil War has never really interested me outside of it being an early example of "modern" fighting on display before WW1. It is not a requirement to have any interest in it to play this game, although I suspect you will probably do a lot better if you are familiar with certain battles. The entire campaign, or rather campaigns (there are two different ones for each side), are a retelling of the war. This is where the best part about this game comes in. Each battle is one individual mission, but they all flow into each other. Your units, their positions, casualties, experience etc. will carry on into the next phase of each battle, and then into the next battle.
If, for example, your best unit gets blown to pieces after you made an error, start the battle again. Sometimes it is actually better to lose a battle and save your troops than to win and have half of them slaughtered, because those troops might not be replaceable with the resources you have and you will need them in the next battle. This creates a sort of multilayered strategic approach to playing the game. You have your immediate goals of defending/attacking, your broader ones such as troop placement/condition in relation to how the later stages of the battle will pan out, but then you also need to think about the next battle and the one after that. All of these things need to be weighed before you make a decision.
The game doesn't have a full release yet so expect more content. Also, if you are going to play this game, consider starting on easy. I had a lot of trouble starting off even on easy, and I play a lot of these kinds of games.
As a result of the continuity aspect of the game, I have actually become attached to certain units/commanders. I can't recall that ever happening before in an RTS, the closest thing I can think of is maybe something like FTL where you have a favourite crew member, but it just isn't the same as UG. Sometimes I will make decisions based on saving a unit I like at the expense of a better unit, even though it makes no strategic sense whatsoever.
I tried the Confederate campaign first, but gave up after I couldn't figure out how to defend the fort in the second mission, so now I'm playing Union. I'm really impressed with the scale so far. Total War rarely had large battles in the campaign, and having huge battles was only possible if you edited the config files and set up a custom battle. I'm also a fan of defensive RTS, and UG does it perfectly. I just played the first day of Shiloh, where you have to defend several different points on the map and slowly retreat backwards as you're holding the enemy off. I thought I'd finish it off before I went to bed, it ended up taking hours and then the second day of the battle started, where you have to push back and secure all the points you retreated from.
Really enjoying it so far, I think my only complaint is that the time restrictions often mean you can't wear the enemy down, you're forced to push forwards instead. I suppose they're just trying to make it more challenging and accurate though.