The vast majority of the fics I read are popular pairings. For one, there's usually a reason why the pairings are popular:) Plus, I read so much that if I limited myself to rare pairings I would have long since run out of fic.

That said, I do enjoy rare pairings fics, mainly *because* authors who write rare pairings often put more time and effort into character development and making the pairing believable. Though there are definitely some exceptions (Chuck authors, I'm looking at you).