This process is purported to be quite thorough:

Each song in the Music Genome Project is analyzed using up to 400 distinct musical characteristics by a trained music analyst. These attributes capture not only the musical identity of a song, but also the many significant qualities that are relevant to understanding the musical preferences of listeners. The typical music analyst working on the Music Genome Project has a four-year degree in music theory, composition or performance, has passed through a selective screening process and has completed intensive training in the Music Genome's rigorous and precise methodology.

From this, an online radio station called Pandora:

With Pandora you can explore this vast trove of music to your heart's content. Just drop the name of one of your favorite songs or artists into Pandora and let the Genome Project go. It will quickly scan its entire world of analyzed music, almost a century of popular recordings - new and old, well known and completely obscure - to find songs with interesting musical similarities to your choice.

I've been listening on Pandora for a few days, and am most impressed. So far, I've tried stations built around Igor Stravinsky, John Adams, Percy Grainger, Lyrics Born, and Eyedea & Abilities. The Stravinsky and E&A stations have been extremely satisfying, the others moderately so. An excellent overall listening experience, especially considering that it is free AND ad free. Check it out.