I'm halfway through a safari with my 1DX2. Having now taken a few thousand frames under quite demanding conditions, I continue to be hugely impressed. The AF performance is outstanding, and it may be a small thing but the focus point illumination makes a massive difference.
While the high ISO performance has definitely improved, shooting raw and in (genuinely) low-light I'm not seeing the ultra-high ISO improvements that some are. To my untrained eye, ISO3200 appears significantly better than on the 1DX1, ISO6400 is a bit better and my personal limit has probably gone up from ISO6400 to around ISO10,000. I do wonder whether some of the really impressive ISO12800 and up shots that I've seen on the web look that good because the subject was well illuminated.
FWIW, another person on the trip is shooting a D5 and to my eye, it does have quite a noticeable advantage at very high ISOs. (And, no, I wouldn't swap).