Supreme Commander is big, big and big, and the firefights provide an explosive experience that RTS fans shouldn't miss.
Well, that isn't completely true, as I do understand how it might be TOO big for some folks, but I personally find the expansive maps to be just what games of this type have needed for years.
There are three campaigns, one for each faction. The United Earth Federation (UEF) fights for law and order; the Cybran Nation fights for independence; and the Aeon Illuminate fights for spiritual enlightenment. The three scenarios (and their associated factions) aren't that different from one another in practice, so it's really just a matter of preference. I like the Cybrans' desire for self-rule, but the Aeon units look the best. The UEF represents big government, so screw them.
The gameplay of SC doesn't add a lot to the genre, but it does take what exists and portrays it on a scale never before seen. Essentially, you start out with your Armored Command Unit (ACU, which is you), and this giant walking vehicle is capable of constructing everything you need to get your base up and running. You'll start out by building mass extractors (for mass) and power generators (for energy), and these are the only two resources you'll collect, as far-future technology allows anything to be created from just two basic components. Later you will construct engineers, land factories, air factories, naval bases, artillery and gun emplacements, not to mention the plethora of units that can be produced at some of those buildings. The middle and endgame will frequently entail swarming armies (up to 50,000 units, I believe), mammoth experimental units that are unique to each faction, as well as devastating nuclear detonations.
And it's all loads of fun.
AI opponents will keep you busy, but humans in multiplayer can make for some utterly ferocious combat moments. With the giant maps, it's not like a typical RTS battle, where you plug a couple of choke points and see who can crank out the biggest horde. Yeah, numbers still matter in SC, but you will regularly find yourself having to mount a 360-degree defense ... or die trying. Air patrols and transports, amphibious assaults, lumbering tanks and walking assault and siege bots -- the scope is sheerly staggering.
Lag can be a problem in huge battles, even on systems that greatly exceed the recommended specs, so it might be a year or two before affordable processing capabilities catch up with the game.
Graphically, Supreme Commander looks really nice on mid-high settings. I have a GeForce 7600 GT with an Athlon 64 X2 4200+, and this rig performs admirably for the most part. On the highest possible settings, the game probably looks amazing, but the visuals ain't worth the resultant lag.
Sound = awesome, though the developers could have added some voices to the battle experience. The campaigns feature frequent communication from your superiors and other commanders, but I've never heard the actual units on the battlefield chatter in the least. They're robots, I know, but if they're smart enough to fight a war, surely they can respond vocally to commands. But that's a small gripe, as the sound effects are great.
Conclusion: buy Supreme Commander. The only reason not to is if you like smaller-scale battles, because one thing SC is not, is small. This game should be one of the leading RTS titles for a long time to come.