I think I have never-ever bought something via a banner. I will always read several comparison reviews first. SO ads don't get blocked in my browser for some reason, but the quality of their job ads is very low (e.g. no salary info in 99% cases) and it only works well for open-source-high-rep type of developers anyway. So basically just a timewaster for both me and an ad-provider. Hence block all-around :).