I recently sold my Canon 50mm f/1.4 lens to buy Sigma 50mm f/1.4. I love 50mm. Or used to. 50mm f/1.8 nifty fifty is the first lens I got after my kit lens+telephoto lens in the package. Oh how I loved it. So, I upgraded to Canon f/1.4 couple years ago. Lately I'm feeling like I'm not quite happy with Canon 1.4 because of its un-sharpness. But I still want 1.4 and 50mm.
I used to have 17-50mm f/2.8 Sigma lens that was perfect walk-aroud lens and stayed on my XSi body for ~90% of the time. When I went FF, I sold it. Never once complained about that Sigma lens. That lens was one of my best purchases too. So I'm going for Sigma again.
I think I got a good copy. Focus is right on as far as my tests go, bokeh is beautiful and 1.4 is totally usable compared to Canon 1.4 or even 1.8.
The only downside is I can't get rid of my feeling that 50mm in general is too wide for my taste on full frame. I am used to tight composition of 50mm on cropped frame or my new fav 85mm of full frame. In fact, I feel like my pics need some empty space, so I should learn to love 50mm again.