I've seen enough plenty of comparison videos, it looks very useful to me. How could it not? Optical image stabilization beats digital any day, video and stills.
Nope, sorry. Optical image stabilization is for photos. You can only move the sensor a tiny tiny bit, it in no way can make up for say walking while recording, or bumps in a car etc. It'll help if you're staying very still but that's it. For video, the 6 and 6+ are basically tied in any kind of moving situation. Digital image stabilization (cropping and matching frames) is WAY better than OIS.
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That seems like it would take a lot of processing power
Well, it does..but we have fast phones.