What I have always liked about search is that it needs to be handled on a macro scale.
G' has been adjusting results based on user-stats for a while now (at least, I am convinced of this). The fact that individual users are getting their own results customized as opposed to all users sharing the same aggregate data should have a seemingly small impact for the average website. Since, if you take all custom SERPs - and then average them out - you should get data matching the non-personalized SERPs.
I'm trying to figure out a way to test this, but I'm flying kinda blind to be honest.