I currently use an Olympus E-520 with an adapted Tamron 35-135 (manual lens from the '80s), and manual focusing is very difficult and hard to get right. Looking in the viewfinder, spinning the focus ring does not really affect the image much and I cannot make out the differences between a sharp and blurry image. The diopter is set right and the only way I can take a sharp image is to use Live View to enlarge the centre and focus carefully.
Even with my kit lenses and AF, I still cannot tell the small differences between whether or not an image is completely in focus or not. However, with a film SLR (even without looking at the split screen guides) I can see quite clearly how sharp the image is. The problem here is that the pictures look fine onscreen, but when I zoom in things gradually get blurrier - at 100% things don't look that good unless I actually use Live View.
I'm now looking at used equipment such as the Nikon D200 (older but still a good camera) and the newer Canon 40D. Both of them cost around the same secondhand, and have suppossedly brighter pentaprism viewfinders. They also have electronic aids (lights) that suppossedly can determine whether or not a manual lens is in focus, whereby my Olympus has none. I also like their better ergonomics and control.
Is MF performance really improved on a "pro" camera? Or is there another way to get good MF without upgrading at all? I use my Tamron most of the time and can't get good results by viewfinder alone unless I stop down my aperture (unacceptable in low light).
P.S. I've seen Katzeye split-screens and while they do they job they are simply too expensive for a simple piece of glass.