From a former Amazon employee:

“Worked at amazon in kindle when KU launched. I don’t care to offer you proof, so take my info or leave it; KU: It’s not worth it. Even the team that developed this couldn’t get their heads around where the value was because it doesn’t include huge numbers of big titles and best sellers. A lot of the content is just from amazons own publishing arm, which is known to create only a few big hits (Hugh Howie, etc). KU is basically a repository or channel for most amazon publishing content that is generally not very high quality. Amazon publishing has been racing to redeem itself because it had overlooked fifty shades of grey which initially came to A-Pub but was overlooked. Since then, A-pub has been willing to produce almost anything from street writers with the fear of missing the next equivalent hit. TLDR: amazons own publishing arm generates garbage content in general; Kindle Unlimited is the garbage dump that charges readers a $10/mth fee.”

From http://www.reddit.com/r/books/comments/2kyx7p/kindle_unlimited_does_anybody_think_its_worth_it/