It is a very beautiful lens. I have had one for a very brief period, while testing against the Leica 180/2 and the Canon 200/1.8 on a Canon 1DsII body. All three lenses exhibit no CA, little distortion, outstanding bokeh, and outstandin color rendition. The differences are basically in focusing distance, sharpness/contrast at open apertures, size/weight, and auto-focus.
In terms of overall image quality, the Canon is still the sharpest/contrasty lens of the three, but by a small margin. Both the Canon and the Leica out-performed the Contax at apertures up to 5.6. At 5.6 and higher it is very difficult to tell the images apart from any of these lenses. On a Canon body of course, you get extremely fast auto-focus, and this is where the other two lenses simply cannot compete.
The advantage of the Leica is that it is almost equivalent in image performance as the Canon, but it has the closest focusing distance (1.5m versus 2.5m on the Canon and 1.8m for the Contax). This can make the difference of actually getting the picture or not, especially in tight interior spaces. And, like the Contax it is much more compact and hand-holdable (both are about one pound ligher than the Canon, and have an integrable lens shade).
As for your original question of "worth it", it depends on what you will pay for it. A new Lecia 180/2 goes for over $7.5K, while a new (if you can find one) Canon 200/1.8 goes for about $5K. If the cost for your Contax is substantially lower than this, then you are getting a good deal.
Lastly, these lenses are valuable because of their speed. The Canon and Leica are both superior at wide-open apertures, so you may want to take this into consideration. However, if you are only shooting on Contax film bodies, there really is no choice. It is still the sharpest/APO corrected Zeiss telephoto in 35mm format. If you want the absolute highest quality Zeiss telephoto for your Contax body, then you will have to buy the Hasselblad Tele-SuperAchromat 300/2.8 FE lens with a converter to CY mount, but this will cost you over $25K, and it is much larger and heavier to deal with.
Wait. It is said that Canon will be introducing this killer lens in an IS version soon ( they have to, Nikon already has launched their 200/2 VR). I had the Canon one currently out of production now. Bought it new 4 years ago. I do not agree it was superior to the Leica 180/2. IMO, the Leica produces more 3D effect and the colors are richer looking.
First, it is an unfounded rumor that Canon will re-make the 200/1.8. It was started by someone who wanted the Canon 200/1.8, but did not want to pay the higher price. Once he obtained the Canon at his price, he became quiet. The Nikon 200/2 is not taking any business away from Canon, and they know it. However, in the unlikely event that they do make a 200, it will be inferior. The reason is the use of HRI glass containing lead and arsenic, which Canon was able to do in the 80s and 90s. These materials are considered unsafe to work with and now prohibited to use, and even h&ered Leica in their desire to top this lens. Leica did a darn good job, and was able to improve it on other ways, but not ultimate sharpness.
Also, be very careful about comparing the Canon 200/1.8 lenses. There was a huge buying frenzy on this lens in the last year or so (and responsible for the above rumor). Many people were dumping poor copies with misaligned elements and even haze into the market via ebay to unsuspecting new photographers entering the digital age with the 1DsII. I know this first hand, since my good friend Son Minh Pham had tested several of these lenses against a brand new one. The beat-up Canon lenses were horrible, compared to the Canon new version. The new Canon lens does have the 3D look, and tested side-by-side with a new Leica 180/2, the Canon comes out on top.
We tested all the Leica, Zeiss, and Canon glass over the last six months (and over $50K). The only lens which was able to out resolve the Canon 200/1.8 was the Leica 180/2.8 at infinity. At closer distances, the Canon still produced the best image.
So, I guess the moral is that you are definitely more likely to encounter a great Leica lens than a great Canon lens. After all, it is the individual testing of each lens that is one of the things that makes the Leica more expensive. The variation of Canon quality is well-known.
BTW, I own both the Leica 180/2 and the Canon 200/1.8, new. They both are razor sharp and produce stunning 3D effect. I tested them in the field and MTF-50 charts. The Canon comes out on top, but again only slightly. I keep the Leica because of that darn good close focusing distance. If Canon could do that, I would sell the Leica.
My lenses were both new and sorry, Japanese "sharpness" isn't the only criteria for image beauty. Leica's well known "Micro Contrast" concept produces superior images IMO. OOF areas are more beautiful to my eye also. But it is Canon's well known "pastel" tendencies in color rendition that has me preferring the Leica optics in general, and this lens category specifically. It's just a matter of taste.
So, imperious declarations concerning subjective lens qualities are an un-resolvable debate.
It is true that Nikon isn't taking any 200mm APO business away from Canon ... Canon doesn't offer the lens any longer.
As to whether an EOS 200/1.8 IS is on the horizon, that remains to be seen.
I have no opinion on the Zeiss 200/2. Never shot with it or even held one.
I agree with you 100% Marc. I didn't mean to make "imperious declarations". In general, I prefer the Leica glass over Canon as well. In fact, the 200/1.8 is the only Canon lens I own. Some people want to know more quantitatively what the characteristics are. Even Zeiss publishes MTF, distortion and illumination data to help folks quantify a bit. I have measured the MTF-50 and MTF-30 awhile ago, so I thought I would summarize my findings. Of course, there is no quantification of bokeh, but subjectively I do like the Leica "better" in this regard as well.
As for color rendition, I also find that Canon gives "pastel" tendencies on film. However, with digital sensors the color rendition is a function of the tonal mapping and the color space. I create my own color profiles, and I match my lenses with the color rendition of my Leica 100 APO, since the reds come out better. I guess in the digital world, mileage will vary according to the color mappings that people like and use.
And, it would be great if Canon could actually come out with an IS version that is as good (or better!) than the original lens. We can only hope. I sure hope they improve the close focusing distance though.
Lastly, back to the original post, the Zeiss 200/2 is an extremely fine lens, and much less expensive than the Leica glass. It produces pictures consistent with the bokeh and color rendition that you expect from Zeiss, so I hope that helps.