I have three hand cameras with bellows from 35mm to 4x5. None are a problem for me. A century ago everyone used handheld bellows cameras without giving it a second thought, and those cameras usually lasted a lifetime.
I'm sitting here with my GF670 in my hands to see exactly how I hold it. In my left hand it looks exactly like the picture from 'jbrubaker' in post #5. The baseplate is resting on the heel of the left hand, which crosses under the camera at a 45-deg angle. The four fingers spread across the bottom edge of the fold-out front cover, which in the open position is now 90-deg out from the camera face. And that leaves the left thumb free to fall directly onto the focusing ring. There are three points of hand contact with the camera body, none of which touch the bellows.
From this cradled position the camera is secure and very responsive, although I do also use a wrist strap with all of my hand cameras just to be safe. Others have mentioned fast-moving grandkids. I can easily see doing that with this camera. And having done kids with TLRs myself, I think this camera would win in that situation.
The viewfinder is a knockout, the meter amazingly accurate, and I would be remiss to not also mention that the electronic shutter is so quiet that you will initially think your camera is broken the first time you press the release. Leicas sound like poorly assembled army tanks compared to this camera.
A few other quirks you may be interested in...
The viewfinder shutter speed LEDs are full stops only (i.e., 30, 60, 125, etc.), which limits the ability to finely adjust manual exposure settings when using the internal meter. You only have one-stop granularities to work with. I think Fuji intended for you to always use the camera in automatic mode. In manual mode I simply use an external handheld meter instead.
The detents on the aperture ring are set to half-stops, not the usual third-stops.
As is already well documented, lens hoods and filters must be removed and the lens must be fully retracted to infinity before the camera will close. The tolerances really are that tight. That means when in use it normally stays locked open for the duration. (So again, keep an eye on that bellows.)
The damping grease on the focusing helicoid seems a bit thin for my tastes. I would prefer a bit more resistance and a bit more holding power. Meaning, if I point the camera up and focus, the lens will creep down slightly with gravity if I don't hold my thumb on the focusing ring. This can be a problem with some pre-focus situations where you must wait a bit before the composition materializes.
That's exactly what happened immediately prior to making this photo. My first couple of attempts with this subject were out of focus until I realized what was happening and corrected it. I've considered sending the camera back to Fuji USA to be re-greased, but have not yet decided if it's a big enough issue for me to go to that much trouble and presumably expense.