the game is a technical successor to the old Total Annihilation games
basically, you have infinite resources on the map, but you are limited by how fast you gather/spend those resources.
armies can go from specialized teams to huge, mixed unit armies.
you get weapons ranging from torpedo launchers, artillery stations that can hit from half a map away (on the larger maps), up to nukes and experimental weapons, which are actually balanced.
in this game there are many, many units and buildings, with the majority of the diversity being in the tech 2 stage. unlike most other RTS's, which offer diversity but only really have one or two useful units, every single unit in Sup Com is useful. its not uncommon to see tier 1 units being used at the ending stages of the game.
overall, strategy and unit cohesion are the important factors, as well as the ability to manage your army while simultaneously building your economy to support your war effort because you can, quite easily, outstrip your resource gathering and be left with nothing to defend yourself.
overall, its definitely worth your money, especially since you can get Sup Com and its expansion relatively cheaply. if you feel that the AI is rather weak, there are mods around that improve it in order to give you a better game experience (like sorins AI mod)
just never, ever play the second one. like the-cnc-that-cannot-be-mentioned, it takes the gameplay ideals of its predecessor and ruins it