Where to start? Take a look at my Canon 135mm review posted here and after reading it, you will know I love the lens and suggest if you want a killer 135mm with AF... this is the lens to grab for a Canon or Sony shooter.
Now... I have to say... as much as I love the Canon 135L, the Zeiss APO Sonnar T* F/2.0 has won me over. The build quality of the lens is stellar. You know as soon as you remove it from the box, Zeiss does quality work. The focusing ring is super smooth; the lens is solid, and it's a little hefty. Not heavy in a bad way, it balances nicely on the camera with a battery pack.
Macro photography is a passion of mine; I figured if I could nail shots with the Sigma 150mm OS in manual focus, the Zeiss should be no different when it comes to taking portraits or just general shooting. This holds true... to a degree. Shooting F/2.0 close with the limited DOF can be challenging, not impossible. One of the advantages with shooting the 5Ds is I can take a couple of steps back, shoot @ f2.0 for a portrait, expand my DOF a little and when the focus confirmation "beeps" the face 9 out of 10 times will be in focus. Shooting up close, I would suggest using live view for critical focus, when shooting wide open.
Lets us talk about sharpness, color, contrast, and bokeh. This lens is as sharp as my macro lenses through the corners and wide open. There is very minor vignetting.. minor. Colors and contrast... Zeiss produces some of the most "netural" tones or correct tones I've seen. Taking portraits, the skin tones look like they should without being warm or cold.
Bokeh... it's F/2.0... this lens crushes backgrounds... extremely smooth... very pleasing look.
One thing which jumped out at me the first time I shot the lens, no chromatic aberration (CA)... the evil purplish fringing. Shoot the Canon 135L or the Canon 85L and you get purple in your specular highlights. Not in the Zeiss... none... nix... nada!
The lens is not cheap... keep an eye out as they did go on sale several months back. This is something unusual for Zeiss from my understanding. The lens right now is $2200 in B&H. You can purchase one in the used markets for around $1600-$1700.
Is the difference in price worth it compared to buying the Canon 135L? I would say yes. The lens resolves images/details better than the Canon; colors are as they should be, it's sharper IMO, and no CA! Again, the Canon is a great lens... you won't regret the purchase. However, the Zeiss, if it fits your shooting style, is on another level. It's an Otus lens without the Otus price. Not joking... it's amazing!
Some shots below from the lens and the Canon 5Ds.
Garage head shot with my wife being kind enough to let me test the lens the first week I had it.
[IMAGE'S LINK: https://flic.kr/p/uW8Tib]Eye by hatch1921, on Flickr
Nice and sharp... great bokeh. [IMAGE'S LINK: https://flic.kr/p/vcjqQd]Butterfly Wonderland by hatch1921, on Flickr
Another example [IMAGE'S LINK: https://flic.kr/p/uXbnei]Butterfly Wonderland by hatch1921, on Flickr
Morning light on a rose [IMAGE'S LINK: https://flic.kr/p/uVxb5C]Morning Light by hatch1921, on Flickr
No CA [IMAGE'S LINK: https://flic.kr/p/tNQX3b]5Ds and Zeiss 135mm Testing by hatch1921, on Flickr
[IMAGE'S LINK: https://flic.kr/p/uL7s1t]100 at f_56 by hatch1921, on Flickr
And one from yesterday... not a macro lens... but it's as sharp and does a great job!
[IMAGE'S LINK: https://flic.kr/p/E8huKs]Emerald Beauty by hatch1921, on Flickr