I'd like to bring you this little tidbit from the Technology Guide, pg. 61, at the end of the Powered Armor description:
Rumors persist of variant forms of powered armor, including suits sized for giants or that can function as vehicles for multiple creatures.
So yeah, after seeing Colossi, Kaiju and one too many Giant Robot-focused video games and animes, I'd like to build a mech as a lost treasure in Numeria... or a super-weapon being developped by a cabal of AI-corrupted androids as part of a mass production for war. The question is that: Do I make it a vehicle or do I make it a creature?
Vehicle: A character could pilot it. However, Vehicle rules don't cover mechs... yet... so it would need a set of custom rules, such as STR and DEX scores, how the pilot's stats affect the mech (mental scores, skills, feats), damage thresholds and attack actions. The upside is that it fits well as an artifact... or a very expensive vehicle and can be fully-customizable. (I know that DragonMech has rules, but man, these are complex as heck)
Creature: As a creature, the mech is a Construct (Robot), thus a sentient being. A character cannot control it unless that is a pilot compartment and that the pilot succeeds a skill check against the robot. Furthermore, AIs can be inserted and can take control over it. Then again, how many times have we seen machine AIs in fiction :P ? Piloting/controlling it would require rules... although it could work like Possession (rule-wise), except that the pilot can utilize the robot's special abilities, regardless of their nature (Ex, Sp or Su). Finally, the CR would be... rather high, considering the size and technology added on top.
So yeah... which option to pick ^_^ ?