I have always loved and enjoyed playing real-time strategy games, and Supreme Commander satisfied my hunger pretty well.
If you like real-time strategy, alien/futuristic themes, and challenging gameplay, you will enjoy this game.
The aesthetics are very good, with nice special effects for the different weapons and units. There are 3 factions, each with a fairly large pool of different units to choose from (land, air, and water). The music and sound is decent.
The controls are typical of an RTS-game - lots of hotkeying and keystrokes if you are familiar with this type of gameplay. The learning curve is moderate to challenging - especially if you are unfamiliar with the RTS genre. The tutorials are very helpful and recommended if you want to get a quick grasp of the game. There are a number of difficulty settings when playing agains the AI (I always start of with a skirmish against the AI whenever I first try a new RTS).
There are two types of resources, energy and mass, and managing/balancing your upkeep of both is critical to being successful in this game. You can actually hit a standstill if you try to build/produce too many things at once and run out of resources. I still have not been able to find a way to cancel/delete buildings in progress, however, you are able to pass construction to free up resources.
The feedback in the game is very good - health bars over each unit, a +/- number indicating how much of either resource you are gaining/losing per second (you can switch this with an efficiency %). There are tabs for each research level when you are trying to build structures or train units. There isn't much detail on each unit type (I am used to seeing things like attack damage, defense, etc). From what I've seen, you are only provided with the unit/building health, type (ex: attack submarine, spy plane), and amount of time to train/build.
The story is pretty interesting, there is a nice cutscene in the beginning of the game that paints the setting for you.
You have a commander unit who can attack, build, and reclaim resources. This unit must survive, and you lose if he is defeated. To give a quick boost to resources, you can use your commander (or any engineer/construction units) to reclaim/absorb materials from things like buildings, forests, and destroyed structures/units. The commander unit can also upgrade his components (chest piece, left/right arm piece, back). For example, you can either give the commander a resource boosting arm (for when you use him to relaim material) or a weapons boost on his left arm (you can remove pieces if you wish; upgrading your commander is very expensive and usually done later in the game).
Gameplay is similar to many RTS-games I've played like Red Alert and Tiberian Sun series - there is a faction with cloaked units (radar sensor building to counter). A lot of the battles are decided by rock-paper-scissors mechanics (bombers to destroy buildings and/or slow-moving artillery, fighters to destroy bombers, anti-air to destroy both, tanks to destroy gun emplacements, etc). I do like the versatility of your commander - you can upgrade him to be more of a combat specialist or a building expert.
Overall, I think you should give this game a try if you are interested in the theme or RTS-genre in general, however, the sequel is out, and for a cheaper price too. That being said, I would still give this game a playthrough - especially if you are an RTS-buff/junkie.