On the 'Texas Photo Forum' website, there is a photo of a Toyo 4x5 camera with a reflex system (similar to looking through an SLR I suppose). Rather large, but possibly convenient in windy or fast moving conditions.
I find the reflex viewer really handy when shooting overhead shots in studio. It's makes you two steps lower on the ladder. I've never used it in the field, but that's just my experience.
The reflex viewer looks cool but I'm married to viewing the ground glass straight up. It's being able to view the image with both eyes, as I view the final print, that is my greatest attraction to lf.