In my experience, it isn’t working as I had hoped. I thought, and I could be wrong on this, that Genius would examine your music library and then produce playlists where the songs fit some sort of logical flow. Would this be your interpretation?
As such, I’ve got Genius doing things like putting Nick Cave’s Mercy Seat directly after Mozart’s Requiem, followed then by Pink Floyd. While making for a somewhat interesting juxtaposition, I can’t see how those songs are related.
I have seen, over in the right sidebar on iTunes, where Genius will suggest music that I don’t have by the artist, and then lots of other suggestions underneath. But isn’t this what the Apple Store thing-a-ma-gig was doing?