I recently had a debate with some colleagues about the effictiveness of banner advertising (which personally I hate - but I also understand they are the means to a free internet).
Here's an interesting article on the great click debate one of them just forwarded to me.
I think my conclusion was these ads are fine if they aren't invasive and are relevant. By invasive I mean in two ways, 1) not in my face - if it sits there being an ad then fine, if an exploding helicopter flies across my screen or a pop up is produced then it can cock off! 2) It respects my privacy, I don't mind if the ads served to me are done so due to my surfing behaviour - just as long as I'm not being tracked or havin bots and malare installed to do so.