Have you even given it a try?
As long as your music is properly tagged it works quite well gathering closely related songs. It works much better than shuffle ever did on any iPod
That's debatable.
We're going on the third year of the "feature". That means, at the minimum, I've synced all the songs in my library well over a hundred times, yet there are still plenty of tracks I can find that yield no results from Genius on my computer's iTunes. Now, call me nuts, but that makes no sense - even if I'm the only person in the entire world with that particular music with iTunes and Genius turned on, which is doubtful given that last.fm lists the most obscure track I could find that fails under Genius with over 200 listeners, it still ought to be able to make some sort of list out of it by now.
Never mind the very shallow nature of the lists and their repetitiveness, or the fact that somehow syncing 1500+ songs to my touch will more often than not give me the "not enough songs" error when I know damn well I could go through my music and probably make a 100 song related list instead of the 25 the iPod based Genius is limited to. The only time Genius never seems to fail is if I accidentally hit the "atomic" symbol and then it screws up shuffling in my already smartlist filtered playlist.
If you absolutely won't use smartlists, ratings, groupings, etc., to do the work yourself, yes, it's better than just a blind shuffle of your entire music collection. But it's still pretty disappointing. Given the huge data pool Apple is working from, their connections with the music business, etc., it not only could be so much more, it should be so much more.