I have really fallen in love with the Ee-S focusing screen for the Canon 5D. This is not a review in the sense that I will try to cover everything, but it is in the sense that I am sharing my thoughts on Canon’s über-accurate manual focusing screen. I have been using it almost totally in conjunction with Nikon lenses. The why will come later, but I mention it now because using it with Canon or non-Canon lenses is actually quite a different experience, and the answer to this question will make a little more sense with that information in mind.
Basically, the matte screen from Canon is made to manual focus wide aperture prime lenses. Since almost all (if not all) cameras display (in the viewfinder) at about f/2.4ish, the depth of field you see is quite different than the actual depth of field on an 85mm f/1.8, 50mm f/1.4, or some such lens. Meaning, you never really know if you are crystal clear where you want to be crystal clear and you just have to trust that the camera focused right where you wanted it.