As a user of the IQ series, 150, 160, and 260, the feedback on the back's LCD, at 100% view is excellent in regards to a focus check, i.e. zooming in to view an image after the shot is taken.
Live View on the CCD backs, well, can't really do much with that at least outdoors. Enough has been written about that.
Live View on the IQ150, excellent, at 100% view. Using a DF+ and 75-150 and 80mm LS lenses, I found the feedback at 100% to be excellent. As I am nearsighted, to really check the screen, means taking off the glasses, and getting close, but this is true for me with Nikon, Canon etc. When using the IQ150 with the DF+ I felt very comfortable with the feedback and did not find many miss focus shots. Of course with current Phase Live View, you don't get focus peaking which would be a nice feature. Obviously you don't need a EVF to use it, as the 645z has it and it's quite a good implementation to me.
Tethering, I only tether to the Surface pro 2, or 3. All outdoors, in the field. The best tool I have found so far, (thanks to the work by Ken Doo and Don Libby). With the Surface 2 zoomed to 100%, you easily get perfect feedback on the focus. I am 100% confident, if I see good focus on the Surface 2 or 3 screen at 100%, the image will be fine on my larger NEC 30" screens.
I have not tried my 15" macbook with retina but over the weekend will try it tethered to see and compare it to the Surface 2 feedback.
Capture Pilot, which is Phase One's app, for both the iPad and iPhone, definitely has an issue displaying an image at 100%, and allowing for critical focus. I have noticed this issue and made comments to Phase for over 2 years, however they have never either cared to fix it, or can. The answer I have always been given was the resolution of the retina screen on the iPad is too much for Capture Pilot to resolve. What you get is just enough blur that you really can't check critical focus on finer items, (trees, grasses, distant buildings etc) when zoomed to 100%. The only iPad that really worked at 100%, was the first generation of the iPad mini without retina. The answer I got back from Phase was that, it was just not possible to make Capture Pilot work with a retina iPad, mini or full size, again this was 1.5 years ago. I have tried the app a few times since and the feedback at 100% is still the same, not enough to determine critical focus when zoomed to 100%. This is unlike the view of an image when viewed at 100% on the Surface 2 or 3, which gives the appropriate amount of resolution.
Paul C