THE STANDARD ISSUE CAMERA FOR PORTRAIT PHOTOGRAPHERS

For stills, THIS CAMERA IS AWESOME! I do not own one but have used it. It resolves my few complaints about the mark II: the autofocus system; and ISO selection controls. In addition, now in 2017, Magic Lantern have just made this camera even better - 4K! ...RAW!...However, I have not really played with the Magic Lantern nightly builds myself, from what I know it is not user friendly and playing it back on the camera was laggy and there seemed to be a huge delay on startup and stop. It is awesome that Magic Lantern made it possible for 5D III shooters to shoot 4K RAW, but it's pretty much just for pro's who are willing to put in the effort to get it working for their shooting style, and if wanting to shoot 4K I would be looking at a Sony. 

All the great features of the 5DII are still relevant here on the III, including the durability, the excellent battery life, and the great menu system.

Tip

Use the "My Menu", the menu with the star symbol. Put your most used features there for quick access. This menu is one of the main reasons to have a Canon.

Here's the main changes from the Mark II:

Improvements

5D III autofocus

NICE! So much more complicated than in the Mark II and so much better once you figure it out! Press the autofocus selector button (top right on the back of the camera) and tap the M-Fn button to tap through the focussing selection options (single-point, expanded, zone etc.). And you can edit the options it goes though, using the menu. You can even set it to use two different autofocus selector modes for the two different orientations (portrait and landscape). So, you can have it set to all 61 AF points in landscape and have it set to a specific zone in portrait mode: as soon as you rotate the camera up into portrait orientation, it switches. AWESOME!

ISO Selection

My second favourite feature: you can set the "SET" button to be ISO, so you hold the "SET" button and scroll the front wheel for ISO settings, I find this much quicker and easier to do this than to use the ISO button on the top. 

Depth-of-field preview button

This is minor but I always wondered why the DOF preview button was in such an awkward place on the EOS 3 and 5DII. ...Now that's fixed.

Any downsides?

Zooming on playback and during Live View

As far as I have figured out, there is only one good thing the 5D II had that they took away from the 5D III: On the 5D II, during image playback the two buttons on the back at the top right (AF point selector, and Exposure Lock) were zoom in and out buttons. Personally, I found that quicker than hitting the zoom button on the left side of the screen. 

Manual focus

Although the autofocus on the Mark III is much better than on the II ...strangely, manual focus did not seem to improve. I don't find focus confirmation to be more convincing here than on the Mark II. 

Other than that, the Mark III seems to be an improvement over the 5D II. An even better camera: the 5DS R, which I have not used, but it seems to be a 5D III with a resolution upgrade. 

5D III recommendation

Yes! For portraits, weddings, kids, wildlife/animals. The reason you get a Canon is for the Canon colour science which is perfect for portraits. If shooting mostly sports, there are cameras better suited for that (including less expensive crop-sensor cameras).  If shooting mostly landscapes, I would recommend a Sony (full-frame), or medium format.