Here’s my take, using a Fuji X-Pro 2 for a while.
The OVF, as stated above, is excellent for seeing things not yet in the frame. This is often helpful for judging composition, or anticipating what is going to happen next. When combined with the small EVF screen (lower right of OVF) it makes precise manual focus a piece of cake when “zoomed” and using focus peaking.
AND, interestingly enough, I’ve found that using a long lens w/ the OVF makes tracking moving subjects somewhat easier. Kind of like a “sports finder” gun-sight type of set up. I’ve effectively chased birds in flight w/ my 100-400 using the OVF and trusting the zone focus. As we all know, with a “though the lens only view,” keeping things framed up becomes a real crap shoot. I’ve also used it successfully with capturing my grandson “at bat” in a baseball game. Short tele, OVF as soon as I’m aware that the ball is entering the OVF (but not yet the frame), I mash the shutter and 9 times out of ten I get a good “contact” or almost ball-contacting-bat, shot. Try that with a DSLR. Your only hope there is “spray and pray.”
The EVF is just what you’d expect, and having it is great for ultra wide lenses, and longer lenses when you need to see the whole frame. Also, when shooting in low light, you can see what the heck you’re doing and focus is mucho more easy.
In the beginning, I thought I’d just appreciate the “reality” of the OVF like I did back in the day w/ my Leica M4 (kind of an homage to “the olden days”). But in practical use, I have found that the various uses, and combined OVF/EVF options really provides some unique capability that neither an ONLY through the lens, or through a viewfinder, can offer.
With the new, smaller f/2 lenses that Fuji has released in the last couple of years (23, 35, 50) and the new small lenses like the Mitakon 35mm f/0.95, and the new Lowa 9mm ZeroD 2.8, the X-Pro 2 really makes for a satisfying and capable kit, having the best of both “ways of seeing” your subject, and some “blended” ways of seeing / evaluating.
Rand