Ok so it's only been 3 hours since my copy arrived but the initial impressions are very good:
1) Focus is out of this world fast and accurate, more so then the 70-200 II which was my previous benchmark of best focusing system
2) Sharpness is incredible - so much so that I think it'll make me watch the shutter speed and/or use tripods even more then before - it's so sharp that you'll be able to tell when you moved a tiny bit, and when it lands perfectly, well it does blow away the sharpness of my 24 1.4, 35 1.4, 50 1.2 and 70-200 II by a decent margin - boy do pixels look amazing with this lens!
3) Feels great on a 1DX, balance works out great, even though it's not a super light lens its a pleasure to hold and use on that body
4) Color, saturation is damn good as well - relieved about this since sometimes sharpness / saturation are a trade-off in optical design, and this lens is nailing both of those
In short - it's looking like this will be my new most used lens!