Performance of the 135 STF
In my opinion, the STF has better bokeh than the Minolta 85/1.4 G (though I've not tried the Limited Edition), the 200/2.8 G APO HS, 100/2.0, the 80-200/2.8 HS APO G (and I assume the newer 70-200/2.8 G SSM), and any other "bokeh champ" you care to name.
Of course, the reputation of high quality glass can tend to become exaggerated, and exceed the actual performance of the lens. In my view, this is not true of the STF. In fact, I'd suggest that the STF's reputation, as stellar as it is, underestimates the bokeh delivery of the lens.
I've read some reviews that hint that the 135 STF creates a slightly un-natural look, exaggerating subjects by rendering the defocusing effect too perfectly -- I'd like to refute that suggestion. Well used, this lens brings subjects out beautifully, but not artificially.
Outstanding. Sharpness is excellent once you obtain proper focus. With a smaller viewfinder, such as on the KM 5D or Sony a100, this can be tricky -- more on that below. I've heard that the (discontinued) Minolta 100mm f2.0 is perhaps able to resolve slightly more detail, but I haven't seen evidence of this.
None that I've been able to observe. The colours and contrast rendered by the STF are very pleasing. If shooting in JPEG mode, on any of the compatible DSLRs (7D, 5D, a100 or A700), images straight from the camera are very pleasing (disclaimer: I haven't used the a100 with the STF).
The STF has never produced noticeable distorsion that I've been able to detect.
Under very difficult conditions, The STF can sometimes show signs of chromatic aberration (also known as 'purple fringing'). This can be seen in the twelfth sample photo - the water bubbles reflecting the sunlight show some minor purple fringing. In almost all circumstances, however, its either undetectable or unobtrusive.