Upload & Sell: Off
not an owner (& probably won't own one until next Spring, if then) but here are my summarized thoughts after about 30hrs of field testing + having gone over all 8000ish images I shot:
strengths: noticeably improved AF consistency & precision in both shooting modes, MUCH improved high-ISO performance (I'd shoot the 1DX up to ISO8000 without much worry, whereas I shot the 1Ds3 at ISO1600 & worried about noise in underexposed images), brighter/clearer/larger viewfinder + shorter blackout times + 12FPS = by far Canon's most responsive-feeling & confidence-inspiring camera, enjoyed having fast dual CF slots & the huge, clear rear display to boot
weaknesses: i prefer the 1.3x APS-H sensor format for my "faster" camera body, no AF at f/8, lack of exp compensation (still?!), new focus point illumination design is silly, at ISO100 i see no reason to "upgrade" from a 5-year old 1Ds Mark III, expensive given the areas where it isn't better than a 1Ds3/1D4 combo, wasn't too used to the new AF point coverages (preferred the spread of points on the 1D4 somehow), battery life is slightly worse than previous 1-series bodies (small, understandable issue)
that said.. the 1DX is without a doubt the single most comprehensively-capable camera body I've ever used & if money were either no object or if i shot indoor sports for a living, I'd buy two of these in a heartbeat.. i'll likely buy one before next wedding season, while keeping my 1Ds3 for ISO100 studio/portrait work & my 1D4 for fun aka birding.
if anyone's interested, my full 1DX review w/ comparisons to the 1Ds3/1D4