I use a few 6x12 backs for my 4x5 and my 3-lens Xpan kit for panoramic. The lenses for the Xpan are incredible, they will really push a film like Technical Pan to the utter brink. So one of my favorite travel kits is the Xpan with the 30, 45 and 90 and my Leica M3 with a 50mm 1.4. Not much you can't do with that setup really.

That being said, I love the aspect ratio of 6x12 and the ease of using rolls film with it and obviously, much larger negatives than those from the Xpan...