Superstar mode is a fantastic, really addictive single player mode. If you intend in playing more multiplayer, I'd get the Wii version (if you have one). The minigames in the Wii version, especially 2 vs 2, make up for the downfalls.
Got to agree with Bowser, don't even touch GoW unless your playing with a mate (unless its a ScottyMate, because he's a prick). It's so much easier and enjoyable in co-op than on your own. CTU Mason and myself recently finished it on insane (after dying about 37 times in the minecart act), there's no way I could have done that alone.