I used a Katz Eye split screen on some previous equipment, and I didnt get on with it. They start to black out slightly the more you stop down. At first signs of black out, you have to re-position your eye in the VF, but even tiny movements of your head position means you see bits of black out. Eventually, at about f8 ( generally speaking ) the black out becomes total in the central focusing portion.
I sold mine on. They're not cheap either, especially for the UK as they're a US company and we get completely ripped off by the UK Customs and Theft dept.
For manual focusing aid, live view would be much cheaper if you already have it, as its free, or use a VF magnifier.