I use a Zacuto Z Finder 3.0 on a 5D3 for stills MOST OF THE TIME. I put the ML 2.0 on if lighting is complex with the life histogram on screen. Actually, since I received it I haven't missed focus with my fast MF primes, using magnification 5 and 10 from the SET button. And I manage to go proper ETTR. Composition is easier as the VF assisting lines don't run at 1/3 and 2/3. And I stopped worrying about blinking or not blinking AF dots.
The reason to use a Zacuto was quite tragic. I had ordered a Brightscreen in August but the man making them died two weeks later, so my order stagnated and got lost (with the money unfortunately which I know consider donation to the family). There is NO brightscreen any more. So, I went the other way and must say: LV has it's big advantages with MF Primes. On the 135 and 85/1.4 I don't use it, subjects move too fast.
Unfortunately, even the well praised LCD is not capable at full resolution to give adequate focusing assistance: the 3x magnification from the Zacuto Z F demonstrates just that I can't rely on critical focus without 5x or 10x zoom. Even the ML peaking is not entrirely helpful as it is diffused. Peaking & B/W high contrast LCD & Zacuto works a dream in videography though. The cam becomes usable for videos suddenly...
I tend to monitor the sensor T regularly as I read here that it gets noisier with higher T. So, I plan the shot a few seconds, switch on LV, press the Zacuto against my (better) left eye, shift the square to 1/3 or middle for composition, have a look at the histo and pik a green ISO value (ML users know about this), go zoom 5x for portraits and more for critical and start shooting. One huge disadvantage is the time lag between a shot and the next one as the machine shows you the taken image, or even if disabled needs time to re-create the LV. But still, it is better than shooting ten times OOF...
Cheers
