One of the studios I work for uses a DSLR for previewing the lighting during commercial shoots. The photographer actually shoots through the 4x5 (back and lens removed) to get a fairly accurate idea of the lighting. Helps immensly to correct hot spots, unwanted shadows, points of tension, etc. Especially during the more intense product shoots, it saves a LOT of polaroids. Nevertheless, for checking the DoF, camera and lens settings, he still does a final Polaroid; but for everything else, viewing a digital preview on the laptop beats the polaroid hands down.