In reply to Denni:
I find it depends on your frame and caliper.
My touring bike has a Tortec Tour rack - not intended for disk brake bikes, but just clears the caliper by a couple of mm. Slightly awkward to adjust the brakes when it's on, but no big deal - solid in use.
Last tour, a friend of mine had a Topeak disk one and had problems with it rattling because of trying to get it tight with the big bosses.