Very high center sharpness, almost unchanged from 2.8 and up
Very good flare control
Sharpness uniformity (especially edges)
Serious landscape, architectural and night photographers will find this lens an invaluable addition to their kit.
Zeiss carefully checks each lens that goes out to meet the requirements and stay within acceptable tolerances, so you can be sure that if you pick up this lens, it will perform.
The front lens cap is felt lined and easy to work with, it requires a bit of force to get off, but that feels more assuring than anything.
The focus ring feels very solid but not too tight, it is easy to understand why this is a very popular lens to shoot video with.
You can get worse flare than this in the middle of the day with the sun high on the sky, but in general the flare control on this lens is stellar.
Take a look at an image captured with the Nikon 24mm F1.4 @ F11:
When it comes to chromatic abberations, yes the lens has some when shooting in very high contrast situations, but it is almost not worth mentioning cause the level is extremely low.
Coma is both an issue and a non-issue depending on what you are photographing and also if you know how to work with it.
Those slight imperfection is noticeable under certain conditions, stars at night, small details around the edges of the frame and so forth.
Have a look at the next one captured with the Nikon 24mm.
The Samyang/Rokinon 14mm is quite a bit sharper overall, but you have to stop down the lens to F4 for it to catch up, and further to F5.6 for it to catch up on the edges, plus it has some hefty distortion.
Here is a shot with the Zeiss on the D810.
Have a look at this image captured quite recently.
I am currently using the Heliopan 95mm 10 stop ND filter with the Zeiss, it has a thin profile and while it adds to the vignette slightly the times you are using such a filter you are generally at low iso levels and stopped down so the effect is minimal.
Singh Ray I know also have some good filters at 95mm, but from the reviews I have been reading it seems that the Heliopan is the way to go with this lens, they also produce a polarizer at 95mm, but due to the ultra wide angle nature of the lens its use would be highly limited.
Here is an image captured without the filter.
At 340$ for the ND and 460$ for the polarizer it is by no means cheap, but I figure if you are going to put your money at a lens like this, you might as well add the filters you need.
When I am out shooting pictures of the aurora being able to quickly unpack my camera and tripod and have perfect focus without fiddling around is invaluable, this lens is pretty much attatched to my camera 24/7 during the aurora season and for very good reasons.
I find the weakness of the Coma and vignette a little annoying, but the disadvantages really does pale in comparison to the advantages of this lens and the disadvantages you can work around anyways.
Before considering buying such a lens I would have to advice to think if you really need it, you can get a stellar performer in the Samyang/rokinon 14mm almost 10 times cheaper, or save 1000$ and get the Nikon 14-24mm.
Here is why I think you should choose the zeiss 15mm over any other ultra wide:
Focusing issues in the dark
Shooting video with ultra wide
Flare/shooting into the sun
Performance in very cold temperatures
After having used this lens for almost 10 months and after many thousands of images it had not once let me down, no matter what situation I have thrown at it, it has handled it like a champ, I would highly recommend this lens.