While I actually like the software (though a nice feature in theory, I personally find Genius pretty silly -- it doesn't recognize half my songs and its playlists are no better than random), you should not install it at the point if you're running Windows Vista.
There are reports that if you plug in your iPod or iPhone when you iTunes 8 on a Vista machine, your computer will crash. See this article.
If you don't have an iPod or iPhone, or you're not using Vista, it should be OK. Otherwise, though wait for the next upgrade.
Oh, and if you've already upgraded, you're stuck. iTunes 8 makes changes to your library so it cannot be read by iTunes 7.