I don't like this kind of thing. I feel like it's fairly disrespectful to the user, just like desktop style hover ads and auto-play videos and anything that covers the screen or locks to a portion of the browser. Of course, it depends on the type of site. For a viral site I'd slam the user as a "stupid tax."
Tested them briefly a week or so ago, but disabled them.
The way I see it, whether or not to use those units will depend on your goals. When I first started my site my goal was ad arbitrage (like many viral sites) to make a profit purely off ads. In that case, those units work well.
These days my goal is more user focused so those page level/anchor ads don't suit me.
I'll say, if part of your strategy is to get lots of social shares via mobile (which is part of my strategy), I don't recommend using those anchor ads. Well, you should test it. But in my own tests the anchor ads decreased the mobile shares. Makes sense since the anchor takes up the prime social sharing area at the bottom.