There's no real violence in the Yu-Gi-Oh! GX world. Duels are fought with a deck of magic cards that produce monsters, lasers, and explosive battles -- but since the monsters aren't real, the violence has no effect. To a non-fan, the duels are ridiculous and surreal. Viewers too young to understand what's happening may be frightened by the monsters and effects, but listening to the characters shout out random and esoteric rules with great drama as they play their cards ("I'm activating my D-shield to protect my Destiny Hero from attack! And if you increase your Elemental Hero's power, my Destiny Hero still will not be destroyed! Ha ha!") might make a watching adult snort her coffee.
The series isbased on a card game. Kids who play the card game will probably like it. Kids who don't won't watch. But compared to a lot of the other alternatives, this is relatively harmless fun, and the game itself is so complicated that mastering it has to count as an achievement, albeit probably not one that's going to help a kid get into college. The characters value learning from experience, playing the game fairly, and refusing to give up. For the right age group, this may not be a show parents will actively encourage, but it's not worth fighting over.