The defocus control is anything but subtle. It gives about the same amount of background blurring as using a 300 2.8 wide open (I have the 300 2.8 Silent Wave for reference). Obviously, the larger aperature you are using, the more pronoucned the effect and just like a long wide open lens, the closer you are to the subject, the more pronounced the background blurring becomes.
If you shoot someone say 10 feet away, and trees and bushes are 10 feet behind them, the bushes and trees become meaningless blobs sort of the way the crowd in the stands become a smear of color in football photos when shot with a long lens.
Let me put it this way, I shucked out $1200.00 for the thing and had every intention of returning it if it didn't work as advertised. That was two years ago, I still have it, and it is my favorite fixed lens.
If I have some time in the next 24 hours, I will try to fire off a couple of shots with the DC on and off and e-mail them to you, but right now I am at home and my camera is up at my office.