Hello all, following this exciting development,
Now that I had the chance to have the 5DMkIII for a few days in my hands, I decided to test the new June 20-th Experimental build from the Experimental download page and thought, it might be useful if I provide some feedback for those who use the 5D3 for shooting RAW video. Here are my first impressions:
Things that I liked:
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1) Compared to the previous March 31 build, this one is very stable. No more crashes, glitches, battery pulls, assert logs, etc. It maintains excellent stability even when I switch modes. One of the coolest features is that it is possible to quickly switch from video to photo mode just by flipping the video/photo switch and the mode dial of the camera from M to Av. This can be done in fractions of a second and every time you switch back and forth from one mode to the other, all settings for both modes are stored and maintained. Thus, when you film video and you quickly need to take a few stills or vice versa you can do this in a very fast and hassle free way without missing important moments. Yesterday, I was filming the performance of my daughter's choir and I did this photo/video mode switching several times during the concert, without a single glitch. Also, when filming FHD video with mlv-lite, I switch from 1920x1080 3x3 to 1:1 all the time. This allows me to switch my EF 35/F2 IS lens from 35mm to 105mm in a very fast way, without swapping lenses or using slow zooms. This is equivalent to the "Movie Crop Mode" on the 100D and yesterday, it worked perfectly on the 5D3 too, without a single glitch. EXCELLENT WORK, A1ex !!!
2) The build is very convenient to work with. In all video modes, in-camera previews work very well and scale nicely when resolution is changed in the 4K-crop recording modes. This is very useful for correct framing during filming. The previews are nice and bright even if bit rates are changed from 14 to 11 ... 8. The feature that I absolutely love is that now focusing at 5x and 10x magnification works almost instantaneously. This feature is extremely important especially when using fast primes for video shooting. A quick and precise focus check and adjustment before pressing the record button is a must and it works very well on the 5D3 now, both - with half-shutter and back-button focusing. I did not have a single misfocused clip during yesterday's filming. Also in-camera playback works quite well and is fully usable, despite the fact that you may end up with a corrupt margin at the bottom and the right of the Live View screen if in the 3K 1:1 mode you reduce the resolution.
3) Another cool feature concerns photo mode with the "HOLD" option activated in the Canon menu. Immediately after a still photo is taken, it stays on the screen as usual but a tiny RAW histogram, focus peaking dots and under and overexposed areas appear on it. In this way, just by taking a look at the picture, you know immediately if it is properly exposed and focused. Very handy!
4) All files that I recorded work flawlessly with MLVFS and open nicely in MLVApp and MLVProducer.
Things that I didn't like:
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These concern mostly my favorite 3K recording mode with mlv-lite. There are two reasons why I use this mode on the 5D3 more often than any other modes when shooting RAW video:
A. The resolutions of 3072x1920 or 3072x1728 (16:9) at 24 fps, 12 and 8 ... 11 bits lossless provide the highest video quality that I have ever seen even after upscaling to 4k. Period.
B. At its maximum horizontal resolution of 3072p, this mode provides a crop factor of 1.87 which is reasonably small and greatly increases the functionality of the available arsenal of wide angle and normal focal length lenses for video shooting. A 50/1.4 Sigma Art lens is converted into a 94/2.2 lens which provides a breath taking vision in terms of isolation of the main subject from the background, bokeh, smooth tonal transitions, etc. At 12 mm focal length, my old Sigma 12-24 provides very impressive landscape videos at 3072x1728 resolution and 24 fps. Fine detail and colors are just fantastic.
There is one major limiting factor of the 3K mode though - the recording time is too short to be usable. With a well lit scene (0.0 histogram value), Frozen LV and 8 ... 11 bits lossless I barely get 60 frames recorded with sound at 24 fps. If I underexpose by one stop, I get about 85 frames and that's it. If the scene contains a lot of highlights, the number of recorded frames can drop to 25 frames which is just 1s of recording time. These data concern the fastest x1066 CF cards. Of course, I could lower the resolution to say 2560x1440 where I can get continuous recording with sound and I do this all the time. However, this increases the crop factor. Also, I could stay with 3072p resolution and reduce the vertical resolution but then I lose this fantastic full-screen vision and the video starts looking like a ribbon. So, what is the solution? In my opinion, there are two ways to solve the problem - through implementing the good old card spanning feature to crop recording with mlv-lite or through overclocking the CF card interface as this was successfully done with the 100D. Or both

I think, more work in this direction is definitely worth considering. A1ex, Danne, what do you think?
Things that need a fix:
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Those concern the sound sync issues discussed above. I hope, they will be fixed soon. I never had any synchronicity issues with the old implementation of ErwinH applied to mlv-lite or the mlv-rec feature from the Nightly builds. So, I guess, going back to these old sound implementations should fix the sync issues, at least for now, until G3ggO irons out his new implementations.
Bottom line:
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The June 2018 builds for the 5DMkIII have turned this camera into a very powerful RAW video recording machine which in my opinion is stable and convenient enough for serious work. After the bugs mentioned above are fixed and problems are ironed out, I am confident that this camera will become the preferred choice for many professional videographers. I wish, I had this stability and functionality in my 7D.
I have to return the 5D3 now and cannot test more features and modes but I will test the June 23-rd build for the 100D and will report on it as soon as I can. I strongly encourage all 5D3 owners in this forum to run their own tests and report their findings here. This will help the developers fix bugs and solve problems easier and faster, so more 5D3 owners can take advantage of this fantastic new functionality and stability of their camera.