My 100 ZF was returned for the same reason as Woody's - the focus action was the worst I'd ever experienced. I mean as in ever, in any lens, new, used, beater or not. Closest analogy I can make was trying to stir a bucket of cement or tile mastic with a paint stir-stick. Unusable.
When I talked to my dealer (popflash), Tony indicated that the stiff focus on 100 ZF samples has been a BIG source of customer complaints - and mine wasn't the worst they'd seen (once they'd sampled the return). This was roughly 9 or so months ago.
Lens was sharp as hell, nice bokeh but CA control not so much. Will also purple fringe easily (when used on EoS anyway). Personally, I'd opt for the 100 APO and the ten-hole leitax mount conversion - under 90 Euros and say 30 min and you'd have the APO in F mount with no adapter BS but with the Nikon D3/700s excellent 'dot & arrows' focus confirmation.
The CV 125/2.5 APO in AiS mount is also VERY sharp and well controlled for CA but I found it a bit slow focusing (not stiff, but slow or fine-pitched) - which can get in the way sometimes for quick non-macro shots if that matters. The CV's price is roughly the same (having climbed significantly of late) as a used Leica or ZF ($1000-1200) but it would give you auto-aperture.
If using for portraits vs. true macro I can also suggest the CV 90/3.5 and 180/4 APOs in AiS. Both are very close focusing (12" IIRC) and sharp as hell starting WO, even at MFD. Focus is stiff new but breaks in to be very smooth with use.
Focus is fine-pitch but my (bought used) 90/3.5 (which I am now looking to replace - arrgh) was easily focused with one finger. The 180/4 is Leica-APO-quality in it's CA control. The 90 is VERY, VERY close but can purple fringe when pushed under the right circumstances. Both are available used for roughly $400.