Naturally MTF charts aren't the basis for the decision, they're just adding fuel to the fire as it were.
Having owned the 35/2 (twice) and 100/2 ZF I have a pretty good idea of what the ZF line delivers re: sharpness, 3D effect, tonality, color-bias (e.g. warm, neutral, cool), etc. Some characteristics vary by lens of course (resolution, vignetting, field curvature, near/far performance).
While I think we agree on most aspect of the (fine) performance of the ZF line Paul, where we disagree is that I consider some of the lenses over-priced vs. new/used alternative offerings (which vary by camera platform of course) w.r.t. CA control, field curvature and mechanical QC (very good, but not as 'premium' as name would suggest).
While mechanical sample variation is not a big deal as exchanges are perfectly doable, it's annoying when you're in that snack bracket - and a PITA cross-border.
As a result I tend to look for alternatives - AND for a select number of ZF lenses on the used market. As a matter of fact, I just missed a nice (non-eBay) 50/2 on the weekend - for $700. So, while still interested in the ZF line (having been there as it were), I'm taking a much more critical eye as to what I'm willing to pay for them.
Having gone thru a no-hold barred Leica (sigh) phase for some time, I've become a bit spoiled as to what premium glass can deliver - but my feelings towards Leica are no different than towards Zeiss ZF - the used market more accurately reflects their (lens-by-lens) performance premium (if there is one) vs. alternatives.
A premium name is nice, and I'm willing to pay a premium price when warranted - but that price has to be warranted. What is 'warranted' of course will depend on what/how you shoot.
As an example, if a premium lens has but average CA control, I'm now paying a greater premium for other aspects of it's performance. If it also has an issue with field curvature 2/3rds of the away across the frame - as WELL AS average CA control, the premium for the remaining aspects of it's performance just jumped once again. Do those other aspects still justify what I'm being asked to pay vs. closer alternatives? If the answer is no, but I simply love the way the lens draws vs alternatives, I wait for a clean used one to pop up.
Sometimes it's too easy to get caught up in wanting to pay for 'the best' (a very loosely defined and subjective term) when if you take a critical eye towards what you're seeing, something 1/2 the price gets you 95% of the way home as it were.
That said, sometimes you just bolt on a lens, fall in love, ignore the price tag and warts and throw the rational analysis out the window. Certainly been there and done that ;>
Now, from what I've seen of the new 60 Micro so far, it's looking stronger and stronger (for me) as a candidate for my 50-60mm walk around lens. Further evidence may indicate to the contrary, in which case, it may fall off the short list.