I'm still using a P25 with a Linhof M679cs, soon to be upgraded to a P45+. A digital back on a technical camera is a powerful combination for certain specialised applications like high quality architectural work, but it brings with it so many downsides that you need to be absolutely sure it's the right solution for your photograph, or else it will be an expensive and frustrating mistake.
Like 4x5 it's slow and cumbersome, but unlike 4x5 you're not rewarded with a unique viewfinder experience or gold standard quality. You're squinting at a 37mm x 49mm image, trying to make precise camera adjustments that will be later scrutinised at much higher enlargements. Image quality is no better (and with the wrong lenses or sloppy technique significantly worse) than you'll get from using the same back on a medium format SLR. And rather than slipping a cheap film holder into the camera, you're gingerly attaching, and then detaching, a very expensive and delicate digital back before each exposure. So check your insurance, scrutinise the weather forecast, and brush up on your sensor cleaning techniques!
Many photographers still carry the conclusions of the film era, where large format automatically equalled highest possible image quality. With digital that equation no longer holds true. Large format digital is really just about extensive camera movements (for moderate camera movements you'd be better off with a Hasselblad Flexbody, an Alpa, Canon T&S lenses, or one of the other more portable solutions), if this is what you need then it's as unbeatable and satisfying as large format has always been. But if you don't need extensive movements then it's an expensive white elephant.
