I've been meaning to compare my fifties, which include the DA 55/1.4 and Tak 55/1.8. The bottom line, though, is that the differences between the lenses aren't very meaningful in photographic terms. I'd recommend an A50/1.7, or DA50/1.8 for autofocus, to anyone who's just looking to make pictures with this length of lens. For absolute value, SMC Takumar 55/2. Personally, I'm overly distracted by minor aberrations that very few other people care about, even if they notice them, and I also am fascinated by the optics and mechanics of camera lenses... but those aren't the same as photography.
Here are my lenses and what I would expect to find in a comparison:
Takumar 55/1.8. Low contrast but sufficient resolution. Good for atmospheric portraiture. Renders veiling glare much like my own eyes, which makes its images more natural and comfortable.
Super Takumar 50/1.4. Similar to the 55/1.8, but with a particular glow or radiance when used wide open.
SMC-M 50/1.7. Solid but not outstanding rendering, in every aspect; which is to say, desirable as a standard lens. Higher contrast than non-SMC lenses, creating an appearance of sharper images. In a word, balanced.
SMC-A 50/1.4. Has the glow of its Super Takumar predecessor, but with much higher overall contrast. Wide open, this lens is all about that glow. Oddly, though technically superior to the Takumar, to me this lens becomes more specialized due to the dominance of that often-desirable aberration.
DA* 55/1.4. Gives what I think of as a modern rendering: where older lenses are desirable for their warm, tender rendering, this lens is desirable for its stark, unfiltered honesty. Basically, this lens' characteristic is that it gets out of the way and lets the subject speak for itself. Plus, remarkably smooth distant background bokeh.
Leica 50/2 (contemporary of the SMC-M line). Avoids the Pentax-characteristic green bokeh fringing. More natural rendering of nearly-in-focus bokeh, at the possible cost of harsher far-from-focused bokeh. Lower contrast than SMC, but likely more center resolution.
But then, I haven't actually done a systematic comparison yet, so that's just my impression. Now I want to see how well my impression matches up with reality. More to come.