I have a love/hate thing for real time strategy games. I love the economics, the fast paced gameplay, and watching my little army men go rampaging. But they require stressful micromanagement, so I never play one for long. My favourite of all time is Rise of Nations.
Supreme Commander is the new big RTS release. It's the successor to Total Annihilation, the 1997 RTS that still has a rabid fan following for its simplicity and modability. I didn't play too much TA, but it looks like SupCom is the same game in every way, just new.
What sets SupCom apart from other RTS games is the economic model. There are a few resources on the map but mostly you build your economy up via buildings and units you can place anywhere. Your economy is all about flow; not "I've accumulated six zillion dollars and I'm going to build a giant army now" but rather "I'm making a hundred dollars a minute and can build 4 giant robots a minute". I don't know the economic model is really that different from other RTS, but it plays better.
The best part of SupCom is the beautifully animated units of multiple scales. It's great fun to watch your spiderbots, mobile artillery, and giant mechasuit guys slug it out. But you tend to spend most of your time zoomed all the way out where the units are reduced to 3x3 icons. But zooming in, there's some fun stuff happening.
It's a good game and I'm already getting a bit tired of it, as with all RTS I play. Part of the problem is that the game is very demanding on computer resources and has a horribly buggy sound system that breaks half the time. That part's not so fun. But the giant robots are awesome. If you want to read more about the game, try this excellent fan blog.