As mentioned above, it is a combination of the small enhancements and extra thought that goes into the design that set them apart. For me, the key differences are the build quality, weather sealing and "toughness". You get the feeling that they'll just keep going and features like the dual card slots give you that extra level of redundancy if anything goes wrong. Image-wise, they're no different to other cameras. In fact, they also miss out on some things, too. For example, the 1Ds Mk III only goes to ISO 3200, whereas the similar sensored (identical?) 5Dii goes to 25,600. Also, 1Ds models lack video. (Who ever heard of a camera that can't take videos?? There was meant to have been a firmware upgrade on 1 April last year which unlocked this feature, but it looks like Canon pulled this at the last minute - probably to get people to fork out more money for the 1Dx).