When we first got the 2012 I thought it was a real PITA to get the mirror adjusted just right to make the mini mirror work for me, and I didn't like the way it took up usable viewing space in the larger mirror,but I have gotten used to it now.
With the 2012 my wife and I were both driving it alot, so we we constantly having to adjust the mirrors to get everything straight for each person. Now that we have our own Mustangs, the mirrors haven't been re-adjusted on mine since the day I picked up my car, so I find the mini mirrors very useful.
My wife still complains about hers occasionally, but that's because I can't quite convince her that she needs to adjust the mirror further out so that she really cant see the side of her car in it like MustangInTexas said.
I have mine set so that the only part of my car that I can see is the very last part of the angle of the fender/rear fascia. This has allowed me to see vehicles in the left lane in the mini mirror all the way to the point my peripheral vision will pick them up with only the slightest turn of my head. On the right side, I still do a "turn head and check" maneuver, but that's 25+ years of driving experience forcing me to do that, lol.