So, I rented the Sigma 14A for my recent trip to Zion. I am still processing the photos. Overall, I am impressed with the IQ. It is certainly sharp from corner to corner (especially f/2.8 on up). What really has my attention is virtually no vignetting at f/2.8 and very little at f/1.8.
I will say, it was a bit of a beast to handle. I felt the need to support it with my left hand if hand holding a shot.
A few nightscapes:
1L0A7736 by kayaker72, on Flickr
1L0A7724 by kayaker72, on Flickr
1L0A7741 by kayaker72, on Flickr