That's a good answer. The 5D Mark III actually did open opportunities for me, really a lot for wedding photography and low-light photography. In these situations, I really need the auto ISO in manual mode and in some cases it's nice to have in-camera HDR and ME. I've been using those features quite a bit lately. I was a 5D Mark II user, so yes, the Mark III improved my photographic opportunities quite a bit.
The 1Ds III, while brilliant, doesn't do well at high ISO's compared to the 5D Mark III. However, it does much better at low ISO's than the 5D Mark III. Both cameras are high resolution, so that pair teams up really well for me.
The 1D Mark IV I love because of faux M mode, and the fps, along with build and active AF spot-metering. It's a great camera for action and sports. I also very much enjoy the 1.3x crop factor.
And last but not least, when I get the 1D X, again, it makes sense for me. Higher ISO capabilities vs. the 1D Mark IV at sports and indoor action, will be a giant upgrade. The two pair very well together, as with the 1D Mark IV I can reach farther and with the 1D X I can shoot at higher shutter speeds in lower light.
All bases and opportunities covered.