Admittedly, I was a little worried when Canon only sent me one battery. The campaign I was on involved a road trip and a lot of mountainous hikes where I'd be in trouble if my one and only battery died. The campaign also involved shooting a lot of 4K video filming which is a challenge for any camera's battery.
I'm relieved to say that I never needed more than one battery per day.
Most days I'd only ever get through half of the battery. The days which involved heavier video work, I'd end the day with 1/4 battery remaining still. I think it'd be more likely that I'd run out of memory on the CFast cards (128GB) before the battery would deplete.
I'd recommend owning two batteries as this would be the most practical (These batteries are relatively expensive - $249.00). Of course you'd have to charge after each day but that's not too inconvenient. The standard charger that comes with the 1DX is also a dual slot charger which works out perfectly.
Video (4K@60fps & 1080@120fps)
The video quality is superb and there aren't many cameras out there that can shoot 4K @ 60FPS with an acceptable crop factor (1.4x). Paired with the amazing dual pixel autofocus system and the touchscreen, the only thing it is really missing is a flip screen. I wouldn't expect a flip screen on this kind of camera though as it would be too big a liability on a camera that is designed to be so robust.
That said, one of the biggest gripes of this camera is the outdated 4K codec (MJPEG). This codec is old and the files it outputs are impractically large. This might be fine if you're shooting an expensive feature film, but it is pretty impractical for any other use.
4K @ 60FPS generates approximately 5.7GB per minute of footage / 171GB per 30 mins
4K @24FPS generates approximately 3.5GB per minute of footage / 105GB for 30 mins
Every night, I'd offload footage and it would take forever. I'd fall asleep multiple times waiting for it to finish offloading. As I was on a commercial shoot, it was crucial that I'd successfully offloaded and backed up my files each and every night.
This issue leads onto my next point:
Memory / Storage
Due to the large file sizes that the 1DX generates, it requires the new CFast memory cards to operate. These cards are great, and they definitely are the future of camera memory, but at this stage they are still quite expensive.
At the time of writing:
64GB costs $591.00
128GB costs $829.00
256GB costs $1249.00
512GB costs $1744.00
I had two 128GB CFast cards which was a good amount to last through a day of moderate shooting & filming. I'd probably recommend getting one 256GB card and a spare 128GB card to start as that would get you through most scenarios.
Another side effect of the file size beyond the two just mentioned, is that you'll need lots of hard drive storage to archive and backup your files. To give you an idea, after 5 days on the road shooting professionally, I racked up about 1.4TB of photos and footage. Imagine the amount of storage you'd require after a full year of shooting. Playback and editing these huge files also require a top of the line (expensive) computer.
Of course a logical solution to this is to be more selective about which video mode to shoot in - lower fps or maybe even 1080p. Given that we're headed towards a 4K future, I feel it is a bit unnecessary to have to constantly decide.
Make no mistake, the 1DX Mark II is an expensive DSLR ($8,699.00) - and that's ok. You've reached for the top shelf camera so you should expect to pay it's worth. I genuinely believe that the price is justified with the DSLR. The quality and reliability it brings to your professional work or business is priceless and you will definitely make your money back in no time.
The form factor is probably one of my biggest issues with the camera. At no fault to the camera design itself, it is purely a clash with my use cases and lifestyle. It is just to big and heavy for my needs. I hiked with it on Mt. Kosciusko for two hours to the Blue Lake with the rest of my gear and it was just far too big and heavy. My back was wrecked by the time I got back to the hotel.