Longstreet- Is a fun game and what drives it is the narrative which is the career of the leader. It's not a simulation game and if your into representing historical battles this isn't the rules for that imho. Now it does have a lot of flavor and delves into the aspects of the period very nicely and if your looking for game to just make you feel like your in the period then this works great, but if your looking to simulate armies on the battlefield and/or re fighting actual engagements then I would look for other rule sets.
Fire and Fury all variants- Great set of rules for fighting scenario's that has stood the test of time. It has several additional books crammed full of scenario's so will keep you busy for quite some time. Rules are pretty straight forward and games flow nicely, but does take some time to play at times. While it can be argued what is the best ACW rule set I think all in all F&F is probably the most played ACW rule set with a great deal of ACW players at least having tried it.
Black Powder- An all around period rule set that is easily the most flexible imho. You can tailor the rules to your collection and how you view the period should be played. The basic rules are very easy and games flow fast. You can change rules or bolt on your own without the rules falling apart. The ultimate toolbox rules that I've very much enjoyed for both my ACW and AWI gaming.
Field of Battle 2- Another all around period rule set that you may really like as it includes cards to decide actions. It gives a very fast playing dynamic gaming experience with plenty of surprises. If your looking for some unpredictability in your gaming then this set is well worth the try. It's a really fun game that deserves much more press then it currently gets. Wonderful!
Your are spoiled for choice in this period and there are a ton more rules out there, but I didn't mention more as I either haven't played them or didn't much care for them and instead just posted what I currently play. Good luck!
Christopher