Ironclad Games's first released title, and a decent RTS which I consider to be at the halfway point between traditional RTS and 4X. It might even be safe to call it a real-time 4X.
As gameplay mechanics go, SOASE is sound, if a bit simplistic. Rock-paper-scissors unit relationships abound, and unit spam is unfortunately a viable strategy in some situations. Each of the three factions is specialized to excel in certain capacities, resulting in widely varying overall strategies and battle tactics. SOASE features hero units in the form of capital ships, which again specialize in certain roles and progress in a MOBA-like fashion, gaining experience through combat, leveling up, and gaining or upgrading abilities (culminating in a powerful "ultimate" ability at level 6). Most battles typically hinge on the survival or destruction of one's capital ships, as they become more and more important targets through leveling.
Technically speaking, combat isn't the ONLY way to win a SOASE match. Like your typical 4X game (say, Civ V), victory conditions can be set which allow for several different paths to win through, for example, cultural influence or technological progress. However, again like your typical 4X game, most matches are won by pure conquest anyway. I don't think I've ever turned on those extra victory conditions since I started playing; conquest is good enough for me.
The Rebellion expansion adds factions-within-factions, with each race dividing in half to offer more specialization, as well as faction-specific Titan-class supercapitals, massive investments in time and resources which serve as the nucleus of an effective late-game fleet.
And that's... really about all I can say. I find it quite fun, both alone and with friends, but if I'm honest, there just isn't enough there to be worth the dollar amount Ironclad is asking for. I understand they're still a fledgling studio that needs the sales to improve and progress as developers, but frankly I believe they're only hurting their prospects by charging $40 when there isn't $40 worth of game. It is, however, a good value for $20 which is the price at which I bought it on sale, and of course any sale price lower than that is even better. The game's capacity for randomization can ensure no two matches are the same, so the replay value on Sins is quite high, but replay value based on randomization is not, in my eyes, a good enough substitute for actual content to justify a price tag approaching that of a AAA title.
Final verdict: definitely worth playing, AND worth buying, if on sale. Don't pay full price for this.