The split prism blacking out on slow zooms bothered me for a long time (and like others, I resorted to using the matte part of the screen to focus on the few zooms I had that weren't AF). But one day, quite by accident, I discovered that if I moved my eye around a little bit in the viewfinder, shazam, the blacked out split prism would go clear. It was a little tricky, but it happened. I also discovered that nearly every time I had already pre focused the lens using the matte part of the screen, the split prism showed that I had missed the focus. Maybe not by enough to make any difference, but it was indeed a little off.
This is regarding film SLRs, which usually (or so I hear) have much better and brighter focus screens than DSLRs. The only DSLR I ever owned, briefly, was a Nikon D50, and it certainly had a darker and squintier viewfinder than my N8008s, which was like looking through a large picture window with lots of eye relief..
And let's not forget focus confirmation lights. Some of my Nikon film cameras had nifty little green lights in the corner of the viewfinders to tell you when you had correct focus.