Andreas: That's the exact switch I made, and I've only 'regretted' it once--when shooting in Arches National Park in Utah in a snow/hail/rain/thunder/lightning storm. The weather sealed nature of the 17-40 would have been nice, and I wasn't sure that the Zeiss would take the weather so nicely.
Shot was taken looking strait up in Jenny Lake Campground in Grand Teton National Park.