Well, I took some photos of random things around my apartment. So far I'm impressed. I shot mainly between 800-25600 ISO, and the camera seemed to perform quite well. Here are some things I noticed...
1. The AF locks on faster than on the 5D2 or 5D3. I heard this might be lens specific, since the 1D X is higher voltage and can drive the AF on the lens faster. But the speed difference is noticeable, at least on the single lens I tested (135 mm f/2L).
2. When you press the shutter, it's loud. It's noticeably louder than the 5D2 or 5D3 (in normal mode). It sounds like you're firing off a gun (ok, maybe not that
The silent shutter feature is still loud. Maybe only slightly quieter than the normal mode on the 5D3. The 5D3's silent shutter is infinitely quieter than the silent shutter on the 1D X, and I kind of already miss that.
3. It's heavy. Granted I haven't been feeling well, and I'm much weaker than normal. But, I got tired after shooting with it for about 20 minutes. I guess it's time for me to hit the gym.
4. The viewfinder is very nice. Very much like the one on the 5D3, but it gives you more information and is nicely laid out (with the exposure metering, buffer size, etc on the right side).
5. I've set it to store RAW in CF slot 1, and JPEG L in CF slot 2. The buffer keeps showing me 16 even when I burst a little. That's pretty cool. And, yes, it does sound like a machine gun
when you hold down the shutter button.