I, too, have been using the 1DS mark3 for the past five years. It was definitely an upgrade when I got my first one. I'd been using both the mark 2 and original 1DS, and, such was the improvement in my images, that I almost immediately stopped using them.
Now, though, I've gotten quite accustomed to the quality of these files. Perhaps a bit too much, as I've lately found myself asking, "What next?"
I've had the opportunity to use the 1DX for a few weeks this past year and am most impressed. Such clean files from high ISOs (1600, 3200, even 6400 was most usable) and awesome autofocusing! It actually made my 1DS 3 bodies, in comparison, a bit disappointing. Fortunately, I never had any problems.
I never even noticed the reduction in pixel count (BTW, it's less than 8% in each dimension) and I frequently produce images that get used double-page size or printed to 40 x 60 (inches). Much as I love the 1DX, I'm still thinking about something a bit bigger. Guess I do have pixel envy!