It's similar to what Pandora radio does, but because it uses your own iTunes library, it has an added benefit for me -- it lets me rediscover tons of music I already own but don't often listen to. I find myself rating lots of songs that I hadn't bothered rating before, and adding some of them to my Favorites playlist. It's just a good thing, and I likes it.
Just one problem so far with the new update -- I can't find a way to get my Library in iTunes to just list all the songs (rather than with graphics by Album, Artist, etc.) the way it used to. Frustrating. Help?
Oh, also when it can't find a Genius option for a song, it tells me to 'Update Genius from the Store menu' -- which is all well and good, but I can't find that option in the Store menu. Meh.