Maybe I'm reading the post wrong. But the OP's subject line says it's not impressive. Not that it doesn't work, or that it's not satisfactory, it states that it's not impressive. Well, it's not impressive. It works ok and can do its job but it's nothing outstanding. The 1Dx's AF on the other hand, is impressive.
My neighbor/cousin bought a 1DX, but I still haven't gotten to play with it much. He's relegating his 5D3 as "the wife's camera"!
Well, I disagree with the premise of this whole thread, because...why does the 6D's autofocus need to be impressive? I would argue that it does not need to be. It needs to simply function well, which it does.
I've never had a problem locking onto objects moving away from me, or towards me. I've never had any problems at all in servo mode. The only time my shots aren't sharp, is when I'm in single shot AF mode, when I should have been in servo (and didn't take the time to multi-half-press).
Frankly, the 6D is an awesome camera for the money. And that's what's impressive! It also feels light as a feather with my 70-200 f/4 lens...where my cousin's 1DX with 70-200 f/2.8 mounted, feels like you're holding three bricks in your hands. He finally bought a monopod, haha...along with something else I suggested he buy...a 300 f/4L. He loves using it with the 1.4x converter...it's a lot easier to carry around than his 600 f/4. He's 73 years old...So now his 70-200 is relegated to event shooting, where it belongs!
I snapped a RAW image at dusk with the above 1DX / 300 f/4 + 1.4x setup, and the chrominance noise at ISO 25.6k, looked about like what my 6D does at ISO 12,800 or 16,000. The luminance noise looked about identical to what the 6D does at the same 25.6k. However, the luminance noise of some of his 1DX shots at lower ISO, around 5000...again remind me of the 5D3. It's larger and coarser in grain size than the 6D's is...I don't care if any of you self proclaimed experts disagree. My eyes aren't lying here. I'm not saying the 6D's sensor is "superior", but I definitely would miss it if I had to give it up and use a 1DX all the time. My point is, the 1DX is not perfect...