I hadn't heard of it, but ran across it tonight while searching for a song. I tried it, and liked it. I typed in the name of a singer that I love and it came up with music that is similar -- based on an analysis "The Music Genome Project", they have done over the last 5 years (and continue to do) of the "musical qualities of each song one attribute at a time.
The website goes on to say "... but now that we've created this extraordinary collection of music analysis, we think we can help be your guide as you explore your favorite parts of the music universe."
I typed in "Judy Garland" and they started playing different artists and asked me if I liked it or not -- based on my answers they came up with a list of artists and songs that contain "romantic lyrics, acoustic piano accompaniment, orchestral arrangement, a lazy swing groove, and interesting part writing". I've listened to Garland, Ella, Sarah Vaughn, Dee Dee Bridgewater, Shirley Horn, Frank Sinatra, Patti Austin, and now Marlena Shaw (hadn't heard her before, but LIKE her sound!) -- without interruptions by other types of music, or having to buy CD's and rip them to MP3's to create a playlist. I'm thinking this is a GOOD thing.
Sorry dial-up folks (I think) - it requires Flash, and probably doesn't work without broadband (I'm guessing).
It's free (if you don't mind putting up with ads ), or you can pay for an ad-free subscription. www.pandora.com