I have the FA 35/2 and the FA 77/1.8 Limited and both are a pleasure to use on a manual focus film body. Not as nice as say my Pentax-K 55/1.8, but still quite nice. The only issues that I have had are:
- The FA 35's focus ring is a bit hard to access when the hood is attached
- The focus action on both lenses is not as smooth as a manual focus lens...you can feel the AF drive through the focus ring
I also have a Sigma Macro lens that is AF, but I seldom use it in that mode. I guess that is comment enough regarding its use as a manual focus lens!
Edit: I forgot to add a note regarding aperture controls. Your MX was designed to use the basic K-mount lenses. As such, you would use the FA series lenses with the aperture ring on other than the "A" position. My FA 35 has a very smooth aperture ring movement. The FA 77 limited is less smooth. This may be an issue with my copy, but to be honest, it feels a little clunky...not what I expected from a "Limited" series lens.
Steve