I have a couple of manual focus lenses that I like to use with a Canon 5Dmk2. In an effort to make this task a bit easier, I'm considering a purchase of the Canon Super Precision Matte Eg-S Interchangeable Focusing Screen.
The specs sheet says to use this with lenses at f/2.8 or faster, lest the viewfinder get "very dim".
I also happen to shoot with some slow lenses, like the 400mm f/5.6.
Does anyone have any personal experience using this screen with slower lenses? Is the viewfinder completely junk or passable in bright light?