In general, I have been very pleased with my Canon 5DIII. However, the one weakness that I have finally addressed is the use of a manual focusing screen. I wanted to let folks here know in case this was an issue for them.
I have several Zeiss Lenses and use Canon tilt-shift lenses that are all manual focus. While the focus confirmation "works" and allows me to achieve accurate focus (especially with the micro adjustment), I find the manual focusing process distracting with focus points and the focus indicator being the prime means of verifying focus when I am concentrating on the image. I longed for a good "old fashioned" microprism where I can simply look at the subject and focus the lens until it looks sharp and the microprisms are clear. This is less of an issue with my tilt-shift lenses as I almost always use live-view with the camera on a tripod. This is real issue with my Zeiss lenses that I focus hand-held, especially the 85mm.
I found a source for focusing screens from a review story linked to the Zeiss website; the provider can be found at <dead link removed by mod>. I believe the gentleman who runs this is based in Taiwan. He custom cuts Canon 1-series focusing screens to fit the 5DIII and includes installation tools and shims to achieve proper focus. In addition, he provides very clear instructions how to install the screen - it is fairly simple, if a somewhat nervous, exercise.
I have been using his microprism screen in my 5DIII for a month and am very satisfied with its performance. I did not need to use any shims as the thickness of the screen is exactly the same as the standard screen in my 5DIII. I have easily verified this by focusing the camera on a tripod and examining the image with magnified live view.
By turning the display of AF illumination points off in the viewfinder (custom settings anyone?), I now have a simple way to configure the camera for excellent manual focus at flick of a switch. It has changed the way I use my manual lenses and greatly enhanced my experience with the 5DII, which was already sky-high.
If you have the same manual focus issue that I do, this could be helpful for you.
A side note: after going through this, I have arrived at the conclusion that the only reason Canon did not provide interchangeable screens on the 5DIII is to maintain enough "separation" between it and the 1DX. I don't make this statement lightly as I have great respect for Canon and its products. However, the removal of the 5DIII screen only requires two screws and is a simple affair. Canon could have provided interchangeable screens for the 5DIII at low incremental cost if they wanted to.. I am thankful that I have an alternative..
I hope this is helpful for others..