I've found FX viewfinders to be much more usable for manual focus than DX ones - this is one place where I DO think that "FX > DX." Part of it is simply that they're bigger (as in 24x36 vs 15x23mm), but part of it is also the optical design. It's just plain easier to get it right.
I have a KatzEye in my D2h and D2x, but I don't feel the need to have same on my D3. (Good thing - they don't offer one for the D3.)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!