The 1D X represents the pinnacle of Canon's DSLR camera technology, and it mostly doesn't disappoint. The body is rock-solid and the controls have been further expanded to work better in both the vertical and horizontal grips. It's also great that the 1D X has slots for two CF memory cards now, instead of one CF and one SD. Only thing we missed was the way the buttons lit up on the Nikon D4 - would have been nice if the 1D X had that too.
Image quality is excellent, if it used to be the case that Nikon had an edge in low-light and low-noise photography, Canon has done very well in bridging that gap with the 1D X. The 1D X does an admirable job of keeping noise down; when noise does appear, it is fine-grained and mostly illuminance noise with little to no color noise. We could very comfortably shoot up to a maximum of ISO 51,200 and still feel comfortable using those images for the web, which is a great help in low-light situations.
Auto-focus is fast and snappy, and the 1D X as well as the 5D Mark III represent a clear improvement over Canon's previous generation of auto-focus systems. But if there's one thing that mars the 1D X as a sports camera, it's the camera's weakness at focusing in low-light with high-speed continuous shooting and AI Servo AF.
The best we can say about the 1D X's auto-focus performance in low-light is that it's inconsistent; in some low-light situations it seems able to focus, in some situations it doesn't, and in some it does focus and then loses focus for no apparent reason. Some users on the internet seem to experience the same problems we did, some don't. The official statement from Canon is that "at 12fps the EV Level is changed to -0.5 EV", which means that its low-light AF capability drops in relation to increasing its shooting speed. The problem is that low-light high-speed shooting is part of the 1D X's mission spec, so you might need to test it for yourself before using it on a critical shoot.
So, in closing the Canon 1D X strikes us as a very, very good camera, among the best DSLR cameras in the world - served up with a qualifying 'but'. If you shoot in good light, the improved handling and AF performance should leave you grinning. If you shoot in low-light, but at low frame-rates or with One Shot AF, then you're perhaps good to go. But if you're shooting high frame-rates of fast subjects in low-light with Servo AF, then perhaps you need to rent and test one first before shelling out the big bucks.