If you browse around this web site you'll see the Adsense advertising on the right hand side of every page. Watch as the advertising changes to match the topics of each web page. Magical, eh? One site I read about Adsense claimed this made for a revolution in advertising, because advertisers could now reach their exact target audience whereas in the past they had to do blanket advertising in hopes that a small percentage of the audience was actually interested in their message.
I don't know about you, but for myself if I'm going to be subjected to advertising as I'm reading a web site, or anywhere else for that matter, I'd rather the advertising be relavent to my needs. What I see while browsing my own sites is, the Adsense advertising chosen for my page usually interests me which tells me it's going to be useful to my visitors as well.