It's noticeably sharper than "old 500". I do have issues with the design of the tripod foot. The balance is wrong for me when using it on my video head. Curious if anyone else has seen this. I haven't read anything about it in any of the reviews.
I also noticed that the center of gravity isn't exactly over the base of the tripod foot. It's more over the "heel" of the foot. I know Canon removed the front glass protective element on the 500mm to reduce weight. Perhaps that combined with the heavier 1DX tilts the balance towards the camera. The balance is further displaced rearwards with the teleconverter attached.
The only place that I've heard this mentioned was on the Really Right Stuff website and regarding their new replacement foot II version.
I was still able to balance the 500II and 1DX on the Wimberley gimbal head, but I was also using a long 6 inch Wimberley plate, which seemed to give me flexibility in adjusting the center of gravity, by sliding the lens forward.
Here's a few picks of the set-up. Sorry about the poor quality, as it was extremely overcast.
Please note that the center of gravity was perfectly balanced as it was positioned in the photos, however I tilted the lens upward and locked the Wimberley head, just for the photos, as these were taken a few weeks ago.
Note that the lens plate is pushed fairly far forward to achieve balance and that the gimbal head knob in the last 2 photos is centered over the "heel of the lens foot and not the base of the lens foot."