I have a copy of Maurice, the previous game in the series, and the rules look interesting but I have yet to have enough games with them to tell whether they will be a "keeper". There are certainly some interesting concepts in them and that's always something that attracts me to rules even if they don't end up having staying power.
The period is certainly of interest but my "go to" set of rules for it is Fire & Fury (the original Brigade level one). Indeed, it has been one of my favourite sets of rules for many years. You can teach them easily and pick them up quickly again after long periods not playing them but they still provide games with a good period "feel" and which are a lot of fun (all completely subjective views I know). So I don't "need" another set of rules for ACW.
I know Longstreet is aimed at a slightly different size and type of game so that might be of interest though.
But what really worried me was that it was simply the "Cult of the New" - the desire to buy a new set of rules (or indeed figures, terrain, accessories, etc.) not because I need them particularly but because it's new. I have certainly succumbed to this in the past and I have significant parts of my figure and rule collections which are a testament to that!
I am also a sucker for presentation and will often persevere with a set of rules which are well presented longer than a less attractive set - how shallow is that? An obvious candidate in this area is Black Powder (and it's siblings) which contain mechanisms which I disliked in other sets of rules but have been willing to tolerate in order to persevere with them.
I am sure another component is my compulsive collecting tendencies (I actually feel uncomfortable if I have a book missing from a series) - although this probably isn't the case here as I don't have Lasalle (the earlier rules in the series) for example. I am currently trying to break this one by not buying supplements for Force on Force for which I have no figures (and it's proving challenging!).
Some of it may well just be my own interest in rules mechanisms. I found Maurice interesting from this aspect and I am sure it has been a key component in my pursuit of SAGA.
Of course it could just be the pursuit of that gaming nirvana - the perfect rules. These are, of course, mythical as all rules are by their nature a compromise between simulation and enjoyment (which are often somewhat subjective in any event - especially the further back we go in time) - and I am sure I could never find a single set of rules which cover multiple periods as I think these will always swing toward the game end of the spectrum and fail to properly scratch the simulation itch for me.
So what's the conclusion? I suspect it's a combination of all these factors to differing extents; however, I have enjoyed the brief navel gazing it has prompted!