Coming from a 5Dc with an EE-S focusing screen, I was pretty disappointed that the 5D III didn't have a similar alternative. Doing some searching, it appeared that focusingscreen.com is currently the only option around. In short, a guy in Taiwan gets original Canon screens and modifies them - there are various microprism/grid/etc screens, but since I like the freedom to manual focus anywhere in the frame, I went with the S Type.
It arrived yesterday, meticulously packaged, and I got to work using their online instructions:http://www.focusingscreen.com/work/5d3en.htm
As I was finishing the installation, I accidentally dropped one of these screws inside the camera, and as luck would have it, it dropped into the hole on the side of the mirror, where I can't see or reach it.
I turned the camera upside down, tilted it, shook it a bit - I could hear the screw rattling around for the first minute, and then it just stopped - I guess it got lodged somewhere.
Thankfully, the part that's held down with the 2 screws seems to be solid enough with just 1, but I'm not really happy about literally having a loose screw inside the camera, just waiting for the right time to break free and go exploring inside a lens... Maybe I should register screwloosephoto.com and capitalize on my mistake.
As for the focusing screen - it works great, same as my previous EE-S screen. To me, it makes the difference between trusting myself with manual focus through the viewfinder, and using it for emergencies only. A bit pricy, but totally worth it in the long term. Despite whatever reason Canon gave for not having swappable screens in the mk III (something about how it's a side effect of their new AF), the various AF modes continue working fine.
If anybody has any ideas on either getting the loose screw out (I was thinking of trying canned air) or at least finding a replacement for it, I'd be eager to hear it.