If you use one without using the other coming from 35mm, it will look as bright and as amazing as you would expect. The 645DF may be slightly less bright then the H4D, and while noticeable if you are comparing them side by side, practically, I don't believe there is a difference, and while I cannot say I love my 645DF viewfinder, I wouldn't say I don't like it, because it just is, its a viewfinder and leave it at that.
Now the H4D, I will say something bad about (and if anyone thinks I'm going on an anti-Hasselblad H series campaign, read my blog posts, I have many things that I like about the series, although more in the lenses), when I was using the H4D-50, I experienced something I had never experienced with any other camera, and have not since then, simply put, it was CA in the viewfinder. I don't know why there was chromatic aberration in the viewfinder, but I found this to be annoying, more so when I was using it for some street shooting tests in harsher lighting conditions then when I was using it later for some landscape and portrait images though.
Thats my thoughts.