The only letter I wrote was to Joe Bob Briggs when he was hosting The Drive-in Theater on the movie channel in the early-mid 90s. I can't remember what I wrote, but I do remember the letter I got back wasn't a completely generic auto-response. I'm still a fan of him and B-movies in general.
I write a email to Leo Laporte when he was hosting the Screensavers that he quickly read and responded to at the end of the show one night, if that counts.