Hi Joanie -- I'm sure I don't answer for everybody, but my experience is that the 65mm frames are easy to see with glasses, plus a nice bit around the outside of the frames too. Without glasses, I can see more, approximating the coverage of the 45mm lens.
The viewfinder is bright with a crisp spot that, like the Leica's, can be used as a split image spot (using the top/bottom edges) as well as a coincident-image spot. Much better than the fuzzy spot in the Fuji GS645 and most other fixed-lens RFs. There's a very clear illuminated display panel at the left if you want to check the exposure settings.
I think turning any camera 90 degrees is somewhat less comfortable than using it level, and the RF645 is about typical in this regard I think. Not uncomfortable or awkward, but maybe a little less steady. But I do quite like its normally vertical frame orientation...
After shooting mostly 35mm cameras recently (testing those doggone eBay purchases!), it was pleasant last Saturday to be using an Olympus Pen half frame (yeah, another recent purchase!), as it too has a normally vertical framing.
Below: See, Bronica and Pen can both shoot horizontals!