I don't have any problems with MF lenses. In fact, I quite often put my AF lenses on manual, although they don't feel as smooth to focus as Zeiss or older MF Nikkor lenses.
So, based on the first few shots through the Sigma 35mm f1.4 any worries about trading one of the best Nikon lenses for a Sigma have gone. This has to be one of the best Sigma lenses ever.