I have two manual focus lens, two auto focus lens and a 6d. I could see in Photoshop focus was often soft or off on a number of photos. Tried a Ec-A (micro prism) and a Ec-B (split image) focusing screen but had two issues: 1) I could not align the small center circle with the center focusing aid and 2) The center focusing aid went dark on smaller apertures especially if it was a low light situation. Solution? I installed the Eg-S. No center focusing aid to align or go dark. I had the more precise focusing I needed without the issues. I do use auto focus in low light with a f5.6 lens. The Eg-S is also not as clear as the Eg-A standard precision screen Canon supplies. Eg-S is designed for f2.8 or larger apertures. I'm using it successfully on f1.8, f2, f4 and f5.6 lens.
The Eg-S is available from Amazon and and other Canon retailers for less than $35. Can be installed on 6D, 5D Mark II and 5D Mark III for sure. May work on other EOS cameras as well. Works for me. I can see far better what is in focus in the viewfinder.