Originally posted by Laurentiu Cristofor That may be the case for many people, but you shouldn't generalize so easily. It really depends on how you use the viewfinder. My manual focusing technique was shaped by the use of rangefinders. When I finally got to use a split prism focusing screen on film SLRs (very late - some time after getting my DSLR), I actually felt it got in the way. My impression is that people use two methods to manually focus:
1 - turning the focusing ring until the area you want to focus on appears to be clear in the rangefinder
2 - scanning back and forth to determine the focal plane position - like you did on old radios when you were manually tuning to a station (another experience that is getting lost these days)
If you're doing 1, you really need a focusing screen. If you're doing 2, you can do without. The difference between the two is in what you're looking for in the viewfinder.
FWIW, my early photography days were always with a K1000 with microprism (no split) and I've found myself going back to the stock screen on my DSLRs. I had a KatzEye in my K20D (I even sent it in to be calibrated after it needed shims), and a Jinfinance screen in my K-7. The KatzEye was nice enough but I never really fell in love with it. I really liked the double diagonal split in the Jinfinance but the ground glass outside the center was much darker than I'd like, and after a couple months, I put the stock screen back. On my K-5, I've just left the stock screen in. I've considered trying the Jinfinance one again (presumably it should fit the K-5 fine) but haven't yet...
Ultimately, the negatives (primarily metering differences, which varied depending on lens and the metering mode, and the inability to see focus anywhere except dead center) did not outweigh the positive (somewhat better accuracy with MF, but even that was no guarantee.) I would love a modern focusing screen with microprism and/or split image, but as far as I know, such a thing doesn't exist...