YGO Online doesn't seem worth it IMO you have to use real money to buy cards. Might as well get one of the Tag Force games on PSP or the World Championship games on DS, there you can buy the cards with in game money.
The makers of the Yu-Gi-Oh! Online game, have recently sent out credits to all whom play it. Something like 100 credits (even though cards are like 40 credits each, it's the fact that they gave out credits). And on the part of the DS games. Already played, beaten, and raped the game with my awesomeness! :3