I use a Katz-Eye with my D70, the first thing I bought after getting it. Because I do not use a split-image, but use the whole screen as matte screen, simliar to the old Type-B screen, I did not opt for the Optibright, as it lowers the contrast of the screen and makes focusing more difficult on the matte screen. If you are going to use a split image, then I would go for the Optibright.
I would not have a DSLR without one, despise the factory screens, completely useless for manual focusing.