The one thing that made me a little uncomfortable was that you to share your library with Apple when you switch on Genius. As I said in Mark Evans' blog:
I’m just beginning to play with Genius, but I definitely have some reservations about sharing my library, especially since many of my downloads are from gray area sources and they “kindly” embed an MP3 tag showing the source that I’m now sharing. In the big picture, I’m pretty comfortable with the big boys keeping my info private, but the dark side possibilities are there !
What I actually did was clear the references to legalsounds and allofmp3 from the comment meta data tags before switching on Genius.
So, Genius isn't really a genius, but it does make some interesting choices and, if nothing else, provides a new way to automatically create pseudo random, but focused playlists. And I'm amazed how fast it works. I like it.