It think it would also be interesting to do with a scale. My roommate just posted a fic yesterday with a purely anonymous poll (hosted off-LJ, and IP locked), where people had options for how much they liked it. She said it was really successful, although it oddly resulted in some people posting comments and saying the poll had guilted them into it. When you are doing it double blind -- we don't know who is commenting and they don't know the author -- I think the results would probably be the most accurate about whether people enjoyed the fic.