On Mediapost Joel Gehman (senior vice president of client services for Refinery) writes an interesting column about the role of rich media in online brand marketing.
Nowadays banners still hold a relevant position in media planning, but in the near future they will be abandoned giving space to higher interactivity, which means rich media will take the lead. As Gehman sees it, rich media create a great brand impacts, banners are no longer able to deliver. On the other side, in the online advertising world, search no doubt works to provide immediate results, that's why he suggests to spend online advertising budget on only two things: search and rich media.
I tend to agree with Gehman, I just would like to add rich media will prove successful if we keep them "simple" in the sense we don't make them too intrusive like, for example, floating ads are. Rich media can create excellent online experiences and deliver the brand messege, but if they become annoying than the negative effect affects the brand perception also.