I know of at least one more who will have both lenses, so I hope we will see posts from more than just me.
So far I have not had the time to do any comparisons. In fact I have not been able to shoot anything of interest with the Sigma yet. But I have had some time to play around with it, shoot nonsense images of door nobs, my cat and other interior details and compare the mechanics, the handling, weight, size, balance etc.
Comparing the build quality of the two is fairly easy, because they are very different. The Otus is heavier, though not much, it is slightly bigger, but both are big 50mm lenses. Not big enough for me to leave them at home for size or weight though. I know I have a different threshold than many others, so I understand some, used to pancake 40ties etc, would have reservations. The difference between the two would not be enough to make me choose one over the other.
Looking at the two lenses side by side, it is very easy to understand that the Otus is a more expensive lens. Its build quality is probably the best I have seen from a DSLR lens. All metal, very smooth finish, beautiful design, classy black anodized topping. No glass and no plastic. But the Sigma is also a very well build lens and if you put it next to a Canon L-series, it has nothing to be ashamed of. The Otus could probably shave off a couple of hundred grams, if it had been using the same materials as the Sigma.
None of them are weather sealed. To me that is a problem. I am used to dragging my L-series lenses everywhere, in any climatic condition. With these two I would feel less comfortable doing that and I´ll probably leave them in the bag on rainy days. The Otus has on several occations been out with me in -20C and brought directly into a warm cabin though. Lots of external condensation of course, but it has dried up nicely and it has not been a problem. I may well be more concerned than I have to.
For natural reasons I cannot compare autofocus. Big advantage for Sigma here. I have bought the dock for the Sigma, so I will go through the AF calibration procedure with it. What I have seen from the AF so far is quite promising.
From a manual focus perspective, unsurprisingly, the Otus is a clear winner. The focus is extremely smooth and accurate. The focus ring moves +200 degrees and this makes very accurate focusing easy. The Sigma is not built for manual focus, so clearly this has not been their priority. But the focus ring moves only about 90 degrees and that makes manual focus more difficult. I suspect that this may also be an issue for AF accuracy, since the movement is so short.
The Otus has a complete DOF scale, whereas the Sigma only show f16. But because the ring movement is so short on the Sigma, it does not make sense to include more (it is about 5mm distance on the focus ring at f16. f4 would be practically nothing).
Both lenses are in my view very well balanced on both the 1DX and 5DIII bodies. There are certain thing you pick up and you feel a kind of solemnity or awe. You feel you have something very special in your hands. The Otus gives you that feeling. The Sigma gives you more of a high quality workhorse feeling. The focus ring on the Sigma is wide, with a nice profiled grip, which is easy to hold and operate. the Otus has a special level, but sticky rubber which I like a lot. The Sigma focus ring is a bit more towards the front of the lens, which I actually prefer to the Otus. But after having used the Otus for 6 months, I have gotten very used to and very fond of how it handles.
So next will be to comparing images. All I can say is that if Sigma proves to have a trustworthy autofocus and image quality close to the Otus, it may be the best news we, the buyers and users, have had since autofocus came.
), but it gives you something to scrutinize. 
