Some are overwhelming or at the very least a bit annoying. The first that pops to mind is space.com - which kept showing a footer ad even though I clicked the "X" - but that's only my immediate example because I'd been to that site more yesterday due to the Mars launch than I have in quite some time.
I noticed that a lot of newpapers seem to have ads that make me want to turn and run away. The full page ads with the "Click to continue" link at the top right. Forbes.com comes to mind immediately. Those ads are repulsive and make me never want to return. And while on the subject, any site that shows a 10 - 15 second video ad before a 15 second video clip will no longer get any of my attention, links, clicks, etc.
If I'm going to continue to try to use the web with ads enabled, I have to draw a line as to what is and isn't acceptable. I haven't tried bitboxer's fork yet, but it sounds like a step in the right direction. To take it further, it might be useful (to owners and visitors) to have a means for site owners to find out whether they are explicitly being blocked and hence losing potential income due of obnoxious advertising.