Canon EF  50 mm  f/ 1.4  USM Lens

Quite possibly the cheapest OEM prime f/1.4 autofocus lens available in any focal length across all current brands. It's fast! The focusing speed/accuracy is pretty good, as is build quality. 

There are 8 blades in the aperture, leading to some seriously smooth bokeh, especially at f/2.8 - f/4. See below for wide-open bokeh notes. 

It's fairly light without feeling cheap or flimsy. It's also somewhat small dimensionally, making it a great grab-and-go lens for a full-frame body. 

Picture quality is quite good, although most copies are not the sharpest 50mm you can get. In fact, the Canon 50mm f/1.8 is often sharper (in the center). 

But, the low-light capabilities of this lens more than make up for its minor shortcomigs.

Weaknesses:

This is one of Canon's oldest lenses in the current lineup. The USM technology in this lens is quiet and fairly quick, but nowhere near as fast or accurate as the current USM ring focusing system.

Build quality is good, but not as robust as the current crop of USM / IS lenses. 

Some may not like the whirly bokeh this lens displays at f/1.4. It's sort of Petzval-like, which may or may not be a good thing, depending on the user and application. If you want to use this as a cheap short lens for traditional portraiture, you might be disappointed that you can't get "plain" bokeh artifacts wide open. It starts looking normal around f/2.8.

Comments:

I learned to shoot with an old Nikon FE and a Nikkor 50mm f/1.4 (AIS). So using this lens feels like coming home. The angle of view is close to "normal" human eye perspective on a full-frame body, and it's ideal for when you can only take one lens but you aren't sure what the lighting conditions will be. It's fast enough to shoot indoors without a flash and still produce reasonably sharp photos. More extensive review by me is here: http://tightcamera.com/canon-50mm-f1-4/