This is a vintage manual prime lens. Optically, this is pretty glass, surprisingly sharp even at f/2, and has nice color rendition. Decent bokeh and reasonably fast. It has a manual aperture ring (no electronic aperture control). It's got a metal body and the focus ring feels amazing. There's nothing wrong with the lens (no fungus, no oil on the aperture blades, no dust inside the lens, etc). Reviews at: http://www.pentaxforums.com/lensreviews/SMC-Pentax-M-50mm-F2-Lens.html

If you want to play around with a manual prime for really cheap, this is a good way to go. (I'm selling it because I got a 50mm f/1.4 and I don't need this one any more.)

It has the Pentax K mount, but you can pick up a Canon mount adapter from Amazon for 10 bucks. Comes with a rear cap, but no front cap.

Attached photos are the top and side view of the lens, a 100% crop at f/2 of a poster in my room, and an example of the bokeh.