All of these codes are reusable because there is some contest associated with them - whoever's code is used the most gets a prize.
Ok, NOW I feel really stupid
The reason I didnt post the code right from the start, was because I thought that its a one-time code, and if someone used it, then some other person might try it, and of course it wouldnt work.
LOL @ myself
So thats really kind of Stardock to give away so many copies of Demigod for 75% off