The "new" old lenses have gained quite a lot of weight compared to their optically same "Classic" siblings. That, for sure, is a big negative.
Personally I don't like the look of the lens, they've overcooked the style.
Calling it the Black Tulip series would have been more appropriate.
I wouldn't want the thing sprouting out of my camera!
I just found a nice detail: The aperture ring for the Nikon version can be "de-clicked" for video. Cute.
This 100/2 macro planar is really a temptation to me. Probably I'll send a note to Santa claus... app. the same weight as the optically not very outstanding 105/2.8 Micro Nikkor, but with a great performance on distant subjects.
Above you said you would want/need a better focus screen for MF? Indeed a nice lens, but I'd prefer 135mm myself. Not for macro though, I disliked 90mm macro on APS-C, not a FOV for me in macro/close up.
Yes, indeed, I'd need a better foucssing screen or a wider focussystem. If I want to use it with Nikon. That's a problem I keep forgetting, too. However, I could borrow the Otus 55 from a dealer connected to the Zeiss tryout program, maybe they have one for the Milvus series, too. They just sent their averts to preorder the lens. Prices are stiff, but they will come down a bit.
The lens itself is tempting anyway. Maybe Zeiss puts a Batis version into their portfolio. Then a Sony A7whatever index they come up with until Christmas could help, as mirrorless systems have some advantage in focus accuracy - nowhere better than on the sensor itself.