I was an A7RII user who upgraded a couple of weeks ago to the A9 (I used the 5d3 before the A7R2). I shoot mostly music videos, live concerts, and a bi-weekly documentary style web series. I know some really love the s-log and swear by it, but I feel I can get just as good of a look by adjusting the in-camera profiles (turn contrast/sat/sharpness down a few notches & adjust white balance) and then color correcting in post. Video editing is my main passion in filmmaking and my strongest area of ability, so I don't mind the extra steps in post to achieve the best picture possible.
I use all Canon glass with the Metabones Canon EF/EF-S Lens to Sony E Mount T Smart Adapter (Fifth Generation).
The A9 thus far has been more than I expected- in a good way. It's FAST when taking photos (sometimes I'm asked to take some during the live show, and the speed of this camera really REALLY helps me get the shot I'm expected to get.) The video quality vs the A7RII is much MUCH better. Less artifacts at higher ISO's and much more clarity/flexibility in post.
Not having S-LOG was never a major concern with me in all honesty, as I said above, I achieve the style & look I want mostly in post with different LUTs. Some will say that's not the correct way to do it for the best picture quality possible, but I'll just agree to disagree with them and leave it at that.
My opinion on the A9 after 2 weeks of in-depth shooting in indoor/outdoor shows with a lot of changing variables in light is it's everything I needed and wanted.
I'm new to Sony and still wouldn't call myself a major Sony guy (it's hard to break away from Canon), but I would buy this camera 10x over given my options at the time of purchase. If you're a guy who is worried about the lack of picture profiles, rent it for a week or 2, tweak the settings to your liking, then see what you can do in post.
Hope this helps someone on the fence about the A9. Obviously if you're on a strict budget, the A7S or A7RII would probably fit your needs better, but if you want to bump your picture quality up and get 120fps @ 100mbps I would seriously consider this camera.