FWIW I've just swapped my 5DMk2 and 50D (and a couple of lenses) for the 1DX. I've never used a 1 series before, so the difference will be more marked for me, but I'm blown away by the 1DX. Apart from it being the heaviest object in the known universe (which I rather like) it really doesn't have a weakness.
FF is great of course. The 1.3 crop sensor was only there for sports photography basically, so the fps rate could be quick enough. With the 2 x Digic 5+ sensors (and the separate Digic 4 sensor for AF) the 12fps @ FF on the 1DX is frankly insane.
The AF is fantastic. Although it has a "mere" 18.1MP, each of those pixels merge seamlessly into its neighbours, so there are no effective noisy "borders" and therefore the low light performance is monstrous. A huge step up from the 5DMk2 ... really huge.
The latest firmware, just out, gives it the ability to AF at f8 now (previously it was f5.6 max) which is very handy for using extenders for BIF etc if that's your bag.
It seems the 1DX really is excellent at everything, with just one exception that I've found. Pocketwizards. If you do any studio or strobist stuff and rely on PWs, they haven't yet got their reverse engineering sorted on the 1DX, so they don't work. They are apparently working on it but it's not a quick fix I believe. Likely to be a couple of months yet.
Other than that, I can't fault it.
For sports, motorsports, kids rushing around, landscapes, portraits ... colours, IQ, AF ... it's got everything.
For landscapes, its sheer imagery and clarity and 3D depth and colour rendition and DR are wonderful
Those who wish to make enormous prints have indicated that a higher MP count would be better, at the risk of sacrificing some of that 12fps machine gun magic (which you may only want to ensure an even better chance of low-light handheld keepers etc) but there are plenty of others who say that using Perfect Resize (as was Genuine Fractals) will upscale any decent image to insane proportions.
Money is certainly an object with me, but despite its cost and the fact that I have to live on beans and eggs for the next 6 months, I'm totally in love with it. Master of all trades, jack of none.