over about a hundred emails i've received about 30 replies.
the surprising thing to me is that their level of 'fame' has little to do with whether or not they reply to me. I've received replies from some famously impressive people, and have been ignored by some real 'niche' ones.
I have a suspicion that there is a bit of a bi-modal distribution between 'small enough to reply to fans' and 'so big that there is a reply crew on-staff', most of my emails that were ignored were "Famous but not that famous" types of folks that are in the middle between the two.