They are really really sharp, after a little AFMA perhaps.
Here is one with no sharpening added yet.
Here is a snap from today as someone asked me is the 50mm was sharp. I think it is... with +6 on AFMA anyway
./showthread.php?p=17467855&i=i23470444
forum: Lens Sample Photo Archive
And one at f/2.2 with some flash
Haven't shot with the 50 Art in a while but used it lots on a shoot today, almost forgot just how sharp it can be.

Sigma 50mm Art up close











