modx seems very new since it's only in version 1.x . Wait till v.2+ before people start using it but I bet it's like another Joomla, Drupal, OpensourceCMS etc... that are out there. You just gotta learn their framework and use it; learn by doing because since it is so new, there aren't any publish books on the topic.
any questions or things you pick up, post it here so that if someone needs it, it can help them.
Good luck man.