Hi all, had my hands on both, lucky me. 1D is as usual 1D pro quality build and will last a pro forever. 5D is, well, like my 7D i suppose. Anyway, what i can tell you is i had my paws on the 1DX for 10 mins going through sub menues and having a talk to the rep. I own a 1DS3 and can tell you the x feels similay, very nice to handle and just feels more solid than the 5. Image wise i know nothing, but i can tell you i was shown the x's new focusing system and was totally blown away by what i saw. Makes my camera feel very dated, which i suppose it is. Its design is very focused on sport/wildlife, anything that moves and needs instant tracking. Its so adjustable to your own specification you can set it for any condition, one setting perhaps indoor track and field, another outdoor sport at night, maybe bird in flight on dull days. Whatever rocks your boat its adjustable for. And get this, they made the menus so easy to access, so quick to set up. Canon have really done the buisness. As for image quality, even the rep hadnt seen anything other than A4 JPEG but said that at ISO 12800 all that could be seen is a tiny bit of noise.
So when you compare 5D3 to 1DX just remember, and here ill use cars, ones a really nice BMW, reliable, quality, handles great, available to all. The others a Bugatti Veyron, all the former and a whole lot more.Is it worth the extra money? Well, if like me you take shots in rain, snow, ice, sand, may drop stuff, do wildlife and sport and need that tough as hell ultra adjustable camera, then yes. If you want a camera that isnt so tough and dont do a lot of sport/wildlife then id stick with the 5.