In my brief testing, the feature was generally good at grouping similar types of tracks, and made for enjoyable playlists, although the mixes were clearly biased in favor of the types of music--rock, pop, hip-hop, classic rock, and new wave--that dominate my music collection. My library contains quite a bit of jazz, classical, and folk, yet because these genres are in the minority, iTunes didn't create a single mix for them.

On the other hand, Genius mixes are like a black box: there's no way to view the tracks in a mix, to edit the mix, or even to delete a mix you don't like.

I also experienced one other glitch that was initially a show-stopper. In order to use the Genius Mixes feature, iTunes needs to update your library's Genius information. This should happen the first time you launch iTunes 9. (If you canceled this process the first time you ran iTunes 9, just choose Store -> Update Genius.) But for me, this process never ended--the progress bar at the top of the iTunes window never finished, and the Genius Mixes item never appeared in the sidebar. I've seen a good number of reports from other users around the Web experiencing the same issue.

As , the problem appears to be caused by particular apple.com entries in your Mac's cookies.plist file. To fix the problem, I had to quit iTunes, open the Security screen of Safari preferences, click on Show Cookies, and delete all iTunes-related apple.com cookies. After relaunching iTunes, my Genius information was updated relatively quickly and the Genius Mixes item appeared soon after.

There's also one more place where iTunes 9's Genius feature appears: Once you've upgraded your iPhone or iPod touch to , the App Store app displays a new Genius screen that recommends new apps based on apps already installed on your device.