As an advertiser I love this feature much more than pop-up adverts, but I have learned from pop-up ads: you have to keep the levels of annoyance below what most people will accept else you will have to stop doing what you want.
position: fixed is great because you can use it to float commercial messages in people’s faces but they can’t do much about it: browsers have pop-up blockers, but not position:fixed blockers. Even totally amateur sites use this for commercial purposes, floating twitter, FB icons etc. in their visitor’s faces. It seems few people have thought this through, the commercial messages and hype seems to have wonderfully side-tracked people.
Best of all though is when people use p:f to recreate frames. But even better than frames the keyboard can’t be used properly to scroll the page, floating elements stand to obscure content. But that just makes the web harder to understand, which tends to be good for business. Bad for the people, but who cares about them? Rich and poor are relative after all, and I’m not going to work to get rich. But I’ll try and push others towards poverty.