I borrowed a Sigma Art 50mm for a while, and it's incredibly sharp, corner to corner. Beautiful pin sharp results.
Hated it, it was very clinical and my images lacked something with it IMO.
The 58 isn't always the sharpest, and before I calibrated it properly it missed focus all the time. But, it's pin sharp in the centre when you get the focus spot on and the images seem to have a lovely almost 3d effect. But mainly, I just gelled with the focal length to be honest, I found it much easier to get the frames I wanted with the extra 8mm. Which is probably my failing as a photographer rather than a success of the lens, but, I love it because I get the images I want from it, with the incredible bokeh.
Would I recommend it to someone else? Dunno, it suits my style, depends whether it will suit yours or not.