I don't really think it's practical though... Why would I want to advertise on a site that has nothing but ads on it. People go there for fun, not to look for something to buy or whatever.
I'd rather advertise on a site that I know has people that would be interested in what I'm advertising. Like webcomics, in my case.
They're probably useless. When was the last time you visited one of these sites? You'd only want to do it if the page has a high Page Rank of 7-10, and that site has a PR of 0.
It was good when that guy first did it because everyone was visiting his page from news stories about it and stuff but with all these other sites it's a bit pointless.