I shall be keeping this one following a fairly exhaustive set of tests both out and about as well as a series of Reikan FoCal AF tests which my copy of this lens flew through very well. As expected there was just the very occasional blip with AF accuracy but the average consistency of focus was 96% overall which is not perfect yet definitely not too shabby however I doubt that there is a lens anywhere by any maker that would achieve 100%. Real-world images showed no out of focus areas, to my eyes at least. Low-light focussing was excellent/perfect with no sign of hunting at all which did impress me. as did the lack of CA which was hard to find.
I do not find this lens to be too heavy at all although it can't be considered to be a lightweight, and it balances up well on my 1DX. Images were found to be crisp and sharp corner to corner even wide open - again impressive. I've attached 2 images of the same (fairly boring) shot with #2 being a heavy crop of the 1st. Taken on a very dull day hence the high-ish ISO.
I have only recently bought this 50mm Sigma as the price has now dropped to around half its original selling price when it was first released, enough to make it very good value for money.