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.