Any evidence that iTunes Genius (incl. Mixes) does what it claims?

By that I mean, is there any evidence that it actually picks "songs that go great together"?

I get the feeling that my mixes are mostly made up of random songs from the same genre (or closely related ones), probably with an emphasis on things I have rated highly.

Is there any indication that Genius looks at other variables, such as tempo, mood, instrumentation, upbeat/downbeat, etc (the way that Pandora does)?