They are a very powerful variant on multiclassing. They aren't meant for normal campaigns, are meant for when the DM wants an extremely high power game or when he wants 2 or 3 characters to have the firepower of a full party.
Take any two base classes that don't inherently conflict on alignment grounds (i.e. you could be a member of both classes simultaneously). You take the best of each class. You get the better Base Attack Bonus of the two classes, the better saves in each category of each one, the better Hit Die of the two, the better Skill Point/Level rate of the two, the sum of both for weapon and armor proficiencies, your class skills are the sum of your two skill lists, and you get all the class features and spellcasting of both combined.
Thus this leads to remarkably powerful characters. Left unchecked, it's a munchkins paradise and DM's are well advised to let them into the game only with very great caution.