I acquired a Pentax M 50mm f/1.7 last night, and it's really impressing me so far. I bought the lens together with a Pentax MX 35mm film camera, a Takumar 28-80mm lens, and a "Phoenix Samyang" 500mm f8 mirror lens all for $80. I really only wanted the lens and will be selling the other items.
Not only is this my first prime lens, it's my first lens faster than f/2.8, and it's my first manual-focus lens. Needless to say, my technique will need a little work. I just brought it home last night, and haven't shot too many pictures yet, but I've already gotten some good ones.
I've really only shot at f/1.7 so far, so I can't comment how it does stopped down, but I'm extremely impressed with the sharpness and overall image quality wide open at f/1.7.
Click on the pictures below for the full size image. All of the pictures are full sized, and the only cropping was on the two portraits just to reformat them to 8x10 aspect. The pictures were shot in RAW, and were developed in Adobe Photoshop CS5 with the normal minor adjustments including minor Smart Sharpening.
And lastly, here's a picture of the lens mounted on my white Pentax K-x: