I have a Maxwell's HILUX screen on 645 system. Feel as a big improvement, and now, can't miss a focus even with my slowest lens. My preferences are the plain screens (without any prisms and other optical helpers), because I shoot a peoples and don't recompose. Have no time, and as you know, the recomposing technique lead focus errors, so I was at need of a screen with excellent abilities to rend maximum detail and contrast off center. Bill Maxwell does for me.
Hope it was helpful.
Cheers!