I just can't help thinking it's overpriced for what it is. The Canon 50 L series can be had for not much more and the optics are excellent on that and it's faster. I tried a 50 L at a show on my 70D and struggled with getting the focus spot on when wide open. It was better with my 5D3 using the small focus points but I don't use/need the small DOF often enough to go for anything more expensive than my Canon 50 f1.4.
Well, the reviews so far suggests the optics are Zeiss league, which is the best lens ever made. That's some claim, how many % that is better than the Canon remains to be seen but on paper, you are getting a £4,000 lens for less than 50L money.
That is a good deal.
Secondly, like you said, you struggle to get focus spot on wide open, if the Sigma is better then that alone is worth it, not overpriced for what it is. You can't put a price no a well timed and in focus shot. You miss it and its gone.
As for not needing the small DOF, that's a personal thing, if you don't need it then you are not looking at any of these lenses so it really doesn't apply here.
This lens is target at those who wants the best 50mm AF lens every made, 50/1.2 or 50/1.4, it's half a stop. I have the 85/1.2 and use that a lot but I never wise my Canon 50/1.4 have that extra half a stop, what I always wanted is better AF (the USM on the 1.4 is not in the same league as an L), and that i wish the lens is bigger. I don't find it comfortable without the hood as it is too short.
As for the grand scheme of things, its around £750 from B&H after Duties and Tax, for a L level quality lens, which is actually really really cheap.