Optics are great, but it's VERY poorly designed.
1. For a 50mm lens, which I use for about 70% of my photos, it stays on my camera for a VERY large fraction of the time. This means it NEEDS to be able to take a beating. Mine has already basically fallen apart, the AF/MF switch fell out, and the focus ring has become completely unusable. Plastic build? Unacceptable. I mean, come on, there are so many cheap 50's made in the past 15-20 years that perform well AND made of solid metal.
2. Autofocus cannot possibly focus a f/1.4 lens accurately enough. Period. Think DOFs so thin that it makes a difference whether you focus on the nose, eyes, eyeglasses, or eyelashes for portraits. Whether or not autofocus is desired at times, manual focus is a fact of f/1.4 photography and should be supported. I need this kind of control from a large aperture lens, and that means focus throws should be long and precise. Complete fail.
After my 50mm literally fell apart I switched to a Zeiss Planar (C-Y adapted to EOS) and have been happy ever since. And the cost isn't that much different from Canon.