The system is basically the same as in all Storyteller games. Only ten-sided dice are used: more of them if the character is more able at what is doing, with the score to be rolled on each higher if the action is more difficult.
Many people find the uniquely freeform magic rules (even slightly more so than [Ars Magica]?) to be a problem, since they can easily be exploited by disruptive players, whilst others find the very same rules a strong point of the game. Also, thematically the problem poses challenges to players and GM, since a large part of the game is about defining what the character think the world actually is.