I've tried manual focus on my Nikon DSLRs and can't get consistently good results. My eyesight isn't what it used to be and the dark viewfinder/lack of split screen makes nailing focus difficult.
I don't find the green focus confirmation dot in the viewfinder much help either.
I have tried KatzEye screens but they tend to make the viewfinder even darker and obscure the image when auto focussing. Guess you can't have your cake and eat it
By comparison, I can MF with my film SLR without any problem at all.