I wanted the sneak attack dice, as I was thinking of going assassin later on, and the ability to brew potions/extracts... assuming I can use the Magus spells to do this...
ie if my Magus spells include Invisibilty, I assume I can brew a invisibilty extract using the alchemist Alchemy.
Also the Mutagen is nice little bonus too, the +4 dex and +2 ac for finessing is nice.