I tested this out last night using Regina Spektor‘s “Us” as my initial song. It served up a great list with Beck, Arcade Fire, The Flaming Lips, Joshua Radin, Damien Rice, Modest Mouse, Band of Horses…
I suppose I’ll have to test it out when I’m in a different mood and see how well it does for that one. But for now I’m pleased.
For my second playlist (that I’m sure wouldn’t be terribly different from the first), I decided to try Feist’s “1234″ since it was one featured in one of Apple’s own commercials. I ended up with this:

Genius is unavailable for this song
I think maybe the Genius cloud was down because it gave me the same error for several tries at songs. I just thought it was pretty funny that the first one I saw was for one of Apple’s own.
Of course they also give recommendations from the store of tracks that will ‘complete’ your library (I’m sure it would never really be complete), but as I rarely listen to music radio (Last.fm if I have some time that I’m not at work as they block ‘Dating and Relationships’ sites there), this may bring some new music and artists to me that I wouldn’t otherwise discover.
It’ll be pretty awesome when I pull a Genius list and it starts recommending Chemtail… first in my list, then in the store sidebar…