I've got both a 1D IV and a 1DX. 70% of the photos I do are wildlife and rock bands. I was really really happy with the performance of the 1D 1V.
The performance of the 1DX is stunning! Let me try and give you some real world comparisons. For birds in flight the IV would track the eye of the bird accurately 5 times out of 10, with the X it's 9 out of 10. When doing low light band shots, the 1D IV needed noise filtering over ISO 1600 , with the 1D X it's 16000 ! The extra shutter speed of the 1DX is welcome, but not actually noticeable in the real world
Loosing the 1.3 crop factor of the ID 1V is a pain for wildlife photography, of course you can crop afterwards, but, I'm constantly reaching for longer lenses that I don't have
The rear screen is supposed to be much better on the X, I haven't noticed a difference
The menu structure, and general firmware capability on the X is much better(More like the 7D, which I've also got)
The X is heavier, but if your are used to the weight of the IV, it's not much different
I don't like the design of having 2 CF cards, rather than 1 SD , 1CF. You can buy SD cards anywhere, and I like the idea of always being able to get a card, if the one I've got becomes lost/corrupted.
Net, net - for low light shots, the X is unbeatable. For fast action shots, the X tracking is outstanding. Everything else is good, but probably not worth the huge amount you have to pay