I am actually a big fan (well...not "fan") of the autobiography. My PS was centered around "why medicine" which forced me to omit a ton of background information about myself and my upbringing. The autobiography gave me a chance to talk about those experiences, lessons, stories, etc, without having to worry about "aw crap this doesn't relate to why medicine", so I feel like admissions will get to see me outside of being premed, my disadvantages, etc.
In short, less than 20% of my autobiography was PS material. Though this is certainly not the rule and I don't know how adcom will react to it.