I fell in love with the lens on my old Rolleiflex camera, a 75mm Zeiss planar, which on a medium-format camera is about the same field of view as this 50mm planar on my 5D Mark II. It has that magical, hard-to-describe 3D look. Super-sharp in the middle, softer on the edges. Wait, is it softer in the corners, or is the contrast just lower? Beyond my technical expertise. But it looks fabulous.
Some say it is soft / not as sharp as one would expect. My bet is that they just didn't nail the focus (more on that below). I don't shoot architecture or other technical work, I mostly do environmental portraits. I want it soft in the corners. So I'm not even looking for edge sharpness.
Also, you can really fine tune the focus - more spin of the wheel to change the focal point compared to a standard Canon lens. Which should make it great for video, although I've only done a little with that.
I suspect that some people try these and then don't keep using them; they do pop up on ebay and other places used quite often. There is nothing wrong with the glass. Zeiss has mastered that. But if you want to shoot wide open, or close, you have to be very good at manual focus. I have 20/10 or even 20/5 vision in my right eye and I still usually take 3 images to make sure my subject is in focus when shooting at f2 or wider. Or that was the case until...
They say the lens "out-resolves" the focusing screen in your camera. That means on a standard Canon 5D Mark II for example, the focusing screen is only capable of displaying f/2.0 or narrower in the viewfinder. You can't see the depth of field of f/1.4. So you really can't manually focus, visually anyway, without changing focusing screens. I installed the Canon EgS focusing screen in my 5D2, a roughly $40 and 2 minute process, and that does make accurate focusing, quickly, much easier. The EgS will show you f/1.0. I recommend trying this screen with a lens like this.
Also, keep in mind that although the camera can't drive the lens to autofocus, the AF points in your viewfinder will still light up when the image is sharp. Pick a focal point, press the shutter button down half way, spin the focus on the lens, and when the AF point lights up, you know you have accurate focusing. This is called Autofocus-assist. Since some cameras, including the Canon 5D Mark III, don't have interchangeable focusing screens, this is the method to rely upon for shooting with a very wide aperture on manual focus lenses.
There is pretty heavy vignetting at f1.4. By f2 its not that noticeable. But wide open you lose almost a stop of light due to vignetting. I think it looks great, but it is surprising at first. I doubt you'll have that effect with a less than full frame sensor.
Bottom line - I love it. The contrast, sharpness, bokeh, and color rendition are amazing. I use this and a 70-200 IS II to cover most scenarios. Both incredible. This Zeiss just takes a little more time, but the results are worth it. I'm in medium format quality territory with the Zeiss - the images are that fine. If I am walking out the door to take photos of I don't know what, this is my favorite lens.
If you are looking at a 50mm I guess the question is which one to buy. This is better build quality and better glass than the Canon 1.4, and obviously more money. I admit, most casual viewers won't know the difference between an image taken with a $350 Canon lens and a $700 Zeiss lens. The Canon 50mm 1.2 (L) is incredible, but I'm not sure it's better in image quality than the Zeiss - maybe, I haven't done a head-on test - but you pay a TON more for the Canon, which admittedly gets you autofocus (a big deal), and from 1.4 to 1.2. $ aside, I like the Zeiss, unless autofocus is a must.
No Canon-branded lens I have tried feels this good in the hand. Zeiss has solid metal everything, weighty, excellent construction.
p.s. I also rented both the 28mm Zeiss and 21mm Zeiss for a few days, and had the same reaction.
Follow Up -
The (all-metal) lens hood has a tension ring of sorts and mine got out of place, making it impossible to keep the hood on. I contacted Zeiss support and they emailed me back in 2 hours with instructions on how to fix it. A small screwdriver and 20 seconds and it was fine again. I would have preferred the problem never happened, but the service and serviceability of the Zeiss product still leaves me very satisfied.