I just noticed that Canon released their new flagship, the 1DX. As an owner of the 1DS MKIII, I was a little disappointed, given the now large choices from Nikon (up to 36+MPs). On the other hand, I'm happy because I won't be upgrading my 1DSMKIII.
I can see if you shoot action, the 1DX might be a must have, for it is superior to the older Canon action cameras. You even get a MP boost and more speed. That would be hard to pass up.
However, if you shoot anything other than speed, I can't see upgrading to the 1DX, especially if you paid 8, 000 USD for the 1DS MKIII in 2008. I shoot models, landscape, interior, autos, boats, studio, products, fine art/artistic etc, and I have never shot speed, although I would like to try some automotive events. Even then, I would expect the 1DS MKIII to do a good enough job. If I shot sports or other speed events exclusively or for my living, I'd definitely go with the 1DX. But precious MPs I'm not willing to give up.
When I upgraded to the 1DS MKIII, I owned and still own the original 5D. The upgrade was was worth it--no difference in noise, 8MP increase, more speed, etc. To go from the 1DS MKIII to the 1DX and lose precious pixels, for me, is a nonstarter. I could never justify the move from a 1DSMKIII to the 1DX, given my needs.
I could see myself going to a new 1Ds series with a good increase in MPs, however, and the option to shoot lower RAW files faster, with all of the new IQ the 1DX has; and that's what I don't get. Why not offer a 28-30MP camera with the option to shoot 18MP files for speed? Why go all the way to 18Ms, and leave out people who need or want as many MPs as they can get? I mean this is a pro camera, so why even add video (replace video with the higher MPs for cost)? I don't really want to pay for something I'm not going to use at the expense of other things I would.
I have to say, if I were on the market to buy either Canon or Nikon at this juncture, I'd pick Nikon hands down--I simply have more options (D800 @ 36MPs) plus action oriented cameras.
Canon's last several years worth of updates have been geared towards those who shoot moving subjects (1D models opposed to 1DS models), while the 1DS line has been stale. I do hope Canon offers a higher MP pro camera in the future. Until that time, I'll be keeping my 1DSMKIII as my primary and my trusty no problems 5D (have yet to need it, but as clean as it shoots, it would do) as a backup. Sorry Canon. It's just not worth it for shooters with needs like myself.