I have the pleasure of being the proud owner of one of the very few Planar's 100/2.0 that were made in West Germany, in the MM version (I don't know how many of these were manufactured before production was transferred to Japan, but I've never seen it anywhere else... all the other German Planars I've seen were AE, and all the MM were Japanese)
This being said, all I can say is that it is one impressive piece of glass. Amazingly sharp at all apertures, sometimes to the point that you have to be careful when doing portraits, the slighest imperfection in the skin will just jump at you! The lens is mercyless in its precision. The color balance is everything you expect from Contax, and its performance in low light is beautifull (why do you buy an F/2.0 for anyways?) It will faithfully reproduce the subtless shades. Use it with a good tripod though!
The only criticisms I have are: if you shoot a very bright subject in a very dark setting (such as a street light in a totally blacked-out neighborhood), then an inverted ghost image sometimes appears, which is probably an artefact of the Planar design. But to be honest I have had this problem only on 4-5 slides over a period of 10 years. The other problem is the weight: it's a hefty chunck of glass. Don't strap an RTS-III with a 100/2.0 over your shoulder for a day-long trek! Also of notice: if you use one of the older Contax bodies (ie. 159MM or RTS-II) you will need that white plastic ring which originally came with the cameras, but nobody knew what they were made for and threw away. That plastic ring is a spacer that you put between the camera base and the tripod, otherwise the lense's aperture ring will be stuck on the tripod base, at an unhealthy angle... If you lost that ring, you can still make a spacer with a thin piece of hardwood (about 3-4 mm thick). Take notice that the 85/1.4, the 35/1.4, the 135/2.0, the 300/4.0 and (I think) the 200/3.5 also need that spacer.
I did not have the chance to compare it side-to-side with the 85/1.4, but from what I've heard, the 100/2.0 has more contrast at full aperture.
Hope this helps