So to confirm, you may use cards from the World Championship decks
- In casual play ("for fun")
- Some Pokémon Leagues (I believe the individual League decides)
I do need to disagree with
@Fayld as even if you are just getting into the game, one or two of these is a good way to see if you'll even enjoy anything beyond the most
casual of play. I am a long time player, still hanging around even though I am critical of the game's current pacing and balance (I just think it can still change). World Championship decks deck to show you the game at its most brutal; if you can't enjoy somewhat (or
very) one-sided games that you'll experience with these decks,
even when you're both using World Championship decks, the highly competitive tournament scene might not be for you...
...which is good to figure out before you invest
serious money building a competitive deck. Which you can get an idea of by looking at the prices of stuff like
Shaymin-EX (ROS).