If you already own Supreme Commander then this title is not really worth the money spent.
The campaign is set a year or so after the Supreme Commander campaigns. Human space has been invaded by an alien force called the Seraphim, and the three primary human factions from the original game have teamed up to combat the Seraphim and drive them away. Essentially the campaign is fairly dry, like the campaigns of the original game, so I'm not going to go over it in detail.
Gameplay wise FA does the standard RTS expansion thing these days by adding new units, and a new faction. While the expansion boasts about it's "over a hundred new units", most of these new units are little more than reskins of old units for the new Seraphim faction. Which brings me to another problem, the Seraphim unit lineup is the exact same lineup found within the original three factions. You've got the light assault bot, the multiple classes of tanks, etc. You'd think that an alien faction from another dimension would have its own unique brand of units, but no it doesn't.
Some other new features include the "template system" which I admit is nice. It allows you to save a building template so you can start a new game without having to go through the headache of going through your standard build order for the hundredth time by allowing the computer to do it for you. Also the user interface has been revamped so that it's less intrusive, and more useful.
Graphically Forged Alliance has had an overhaul making it more graphically pleasing than the original Supreme Commander, but also ensuring that you need an even Godlier computer than the recommended one for Supreme Commander. Honestly, who's coming up with these stupid ideas over at Gas Powered Games?
Audio wise the game features some new music, and to be honest I didn't like it as much as the original Supreme Commander's musical score. It's not bad, but I don't feel that changing it was necessary. I'm not actually sure if the new score is composed by Jeremy Soule who did the original Supreme Commander musical score, but it's solid despite its inferiority.
Personally, I found Forged Alliance to be a waste of money as it doesn't really offer anything worth while to people who already own Supreme Commander. A dry campaign, and a new faction that doesn't really offer anything actually new. If you don't own SupCom, then I suppose that it can be a worthwhile purchase if you don't mind the fact that you can't play as the UEF, Cybran Nation, and Aeon Illuminate online without it.