Since you're on the subject, I'd like to point out that MusicMatch uses a very sophisticated recommendation engine to determine relationships between artists, albums and songs. We do this through anonymous play log uploading.

If a user is opted-in, we upload their MP3 playlists to the server. The server then compiles lists of similar artists based on what is in the playlists.

Basically, you can assume that the 'typical' listener likes a particular style of music and tends to only listen to that. You can also assume that they will listen to similar types of artists back-to-back. So, if a user listens to Pearl Jam and then Temple of the Dog, you can 'guess' that a relationship exists between these two artists.

Now, this process is definitely not perfect. What if you get an eclectic listener (like most of the people on this board) who listens to Pearl Jam, then Bach. Or, as you specified, everybody has different tastes and they don't always 'jive'.

Well, as we all know, in small groups, deviations will be much more apparent. But, when you take into account the sheer amount of play log data we get (millions a month), these deviations become smaller and less noticeable. This is one of the key reasons why our data is pretty good.

For example, the top recommendations for Pearl Jam are:
Soundgarden
Stone Temple Pilots
Temple of the Dog

Those are pretty dead on. Now, admittedly, recommendations for obscure artists won't be as good because we won't have as many play events for them.

But, if you're curious to check it out, here's a link to our Guide, which has a search box that you can enter an artist (and optionally, an album) and get instant recommendations:

http://www.mmguide.musicmatch.com/match/best.cgi