I had 2 1DMarkIV. And I was delighted. But - and this was the reasen why I sold them after approx 3000 shots:
That format APS-H is a big disadvantage using wide angle lenses. You simply loose space. a 28mm lens is only a
37 mm lens. I bought 2 EOS 7 D. They are only APS-C format but the lenses are built accordingly and the camera is excellent. I am waiting for a 1DsMARKIV or whatever the camera will named. But it must a FF Camera. I am very interested if there will be the new model in august.
That doesn't make a lick of sense to me, going from 1.3 to 1.6 crop - wat? The 16-35 is plenty wide on a crop and was more or less made to cater for the crop cameras. People have just gone completely overboard on what is wide angle. A 50MM is more or less considered a tele these days...
Regarding topic - this has been said:
The 5D II is a great camera for what it's good at. But it fails hard in some pretty crucial areas compared to the 1D IV.
Autofocus, speed, build quality = the things that the 1D is. It cannot match it. My 1D II is better in those respects.
Where it wins is the smaller, less threathening size.
Regarding image quality: The 1D IV is less noisy than the 5D II, especially for video.
And they sell plenty. Just got back from sporting event, all 1D IV's or Nikon D3. Same as last time. And the time before that.