How much does your eyepiece cause compromise?
The exposure meter/shooting information at the bottom of the viewfinder can be so dim outdoors depending on the angle of sun it can't be seen. At least on my 300D, that was never a problem - actually was the brightest thing in the viewfinder.
Otherwise, the image of the subject itself in the 5D viewfinder is nice & big. I've bought my first MF lens actually. I also bought the Angle Finder C which does several things: a.) its basic job allows you to place the camera close to the ground and use the viewfinder without laying flat on your stomach on the ground, b.) magnifies 1.25x and 2x, c.) allows me a way to use the expsoure meter on top of the camera easily in lieu of the in-viewfinder meter.
The Angle Finder C fits kind of loosy-goosey though.
The state of the viewfinder in all 35mm cameras seems to be less than perfect. That's understandable to a degree because of the varied conditions of humans and their ability to see. It would seem prudent on the part of the camera manufacturers to provide a chapter in their owner's manuals dedicated to optimizing the viewfinder. Of course, that would involve having to admit that their viewfinder has limitations.