It's a 50mm f1.4... it's sharp enough at f1.4. Gets better at f2 and keeps getting better as one stops down. It's sharper then any zoom for sure.
It's nice and light, but that's because it's mostly plastic. The build quality leaves something to be desired, it feels like a nice consumer zoom, better then the kit 18-55, but not even close to L glass.
Paired with my 5D MKII, this lens works wonders in the dark, IF you can get it to focus. The AF on anything but the center point is hit or miss in the dark. The lens often misses focus and then freezes, I have to put the lens in MF, move the focus ring around, put it back in AF and then it'll work. Absolutely unacceptable. The AF tracking is slow and can't be trusted.
Apart from expecting better build quality for a $350 50mm, I would expect top notch AF as it needs to be accurate at f1.4... it isn't accurate and freezes when it can't lock focus.
Maybe it's just my copy, but it was disapointing. It's the only lens that will alow me to shoot in such low light, but I'm constantly frustrated at it's shortcomings.
For accurate AF in hard situations (dance, concerts, show, indoor events) I ditched the 5D MkII and the 50mm altogether, and use my Nikon gear when I need to shoot under hard conditions. The 5D MkII and 50mm are great for slow paced studio, portraits and photojournalism, but I'll take something else for dance performances and concerts...