I was about to start my own thread about this but since you did...
Library size: 589 songs
Alternative Mix (Coldplay, U2, Muse)
Electronic Mix (Fatboy Slim, sniffies, Röyksopp)
Rock Mix (The Verve, LCD Soundsystem, Scissor Sisters)
Rock Mix 2 (The Who, Queen, Thin Lizzy)
I like how it divided the Newer Rock from the Classic Rock. That said, I wished it would let us make our own never-ending mix (à la Genius Playlist) since I feel a few of my library's genres are not being represented.
EDIT: That's an odd name for a mix: "XM Radio Mix," tkermit. What did iTunes put in there?