The Chaos Space Marines only look similar to the Space Marines for the first tier. Both play differently unless you're an idiot.
And the game does indeed kick ass, it was quite possibly the best RTS of 2004 (I never played Rome Total War, but I heard it leans on the single player side of things). The resource system encourages lots of action from the get-go and no messing around with peons and whatnot. Throw in the victory conditions that revolve around control areas of the map, and the game is without action.
If the campaign was beefier, and some of the audio and graphic glitches were tightened up (though the voices aren't as bad as Heald would have you believe, the Orks as a whole kick ass), I would call this Son of Starcraft.
And if you want to see a good death animation, try to get a Force Commander to land the killing blow on a Bloodthrister. Just awesome.