Nikon Rumours has posted a comparison with the Nikon 50/1.8
This came as a surprise because I didn't realize that Yongnuo was now producing glass.
The Yongnuo didn't exactly blow the doors off the Nikon, but it is so cheap, who cares? However, the old 50mm/1.8 D is already so cheap, that buyers might not mind paying a bit more for quality glass.
Still, this might be like Yongnuo speed lights: It's so cheap that if something breaks, you haven't invested a fortune.