So I was eventually able to download iTunes 8, which, as noted, intrigued me most for its Genius recommendation feature. So how did the Genius do? I have over 7,000 songs in my music collection, though it maybe isn't as genre-spanning as that number might indicate (there's nothing by way of jazz or classical, say, 'cause I'm a philistine). I also don't bother to rate individual songs or futz with whatever "genre" tag Gracenote assigns even if I think it's off.
The Genius playlists generated for me seem for the most part to be logical. Building a list based on French techno-pop band Air gave me stuff by the Postal Service, Portishead, Tricky, and Fatboy Slim, which made sense, though there were others that seemed a little further afield (Suggie Otis? really?), and other bands (like Stereolab) that I expected to see didn't appear. Going to the other end of the spectrum, Led Zeppelin's "When the Levee Breaks," brought me relative contemporaries (Jimi Hendrix), possible musical descendants (Soundgarden), intriguing near-misses (Nine Inch Nails), and just plain head-scratchers (Devo).
Most other reviewers seem to have the same mixed opinion of the actual implementation that I do (see for instance CNN and the LA Times), but their enthusiasm for the project as a whole varies based on what they think the ultimate aim is. Of course, Apple's real not-so-secret agenda is not to give music fans a chance to endlessly analyze how their music collection fits together (or doesn't), but to push more iTunes Store sales, via the Genius sidebar. When you highlight a song, the "Top Songs You're Missing" feature tells you the most popular tracks by that artist that you don't own, which is obvious but very clever. Then there are the recommended tracks from other artists, which run the same gamut as the Genius playlist does in terms of reliability. It's pretty slick, and may make money off of people who are itchier on the buy-it-now trigger finger than I am.
One of the tech bloggers over at the UK's Guardian goes as far as to say that this is Apple's equivalent of music subscription service, in that it provides a steady stream of revenue from eager music downloaders while still giving users real songs that they really own. And don't worry, rumors of an actual subscription service still haven't died; it's totally coming in iTunes 9 now.
(Oh, and one last tip: if you really like a particular Genius playlist, you can save it.)