Steve,
I personally now own and have used the following Yashica lenses quite a bit in the ML/MC line:
Primes:
24/2.8 28/2.8, 35/2.8, 50/2, 135/2.8 (compact version) and 200/4. I like them all, but LOVE the 24mm. I have no Zeiss equivalent to test it against however.
All of these have been very good when compared to my Zeiss lenses of the same focal length, with the exception being the 50/2, which has, in my opinion, neither the build quality of the Zeiss 50/1.7 or 1.4, nor quite the "pop" of either of the Zeiss normals. Across the board, to my eye, the Zeiss lenses seem to render scenes a little warmer than the Yashicas (red is vibrant), but the Yashicas are perfectly acceptable. Generally from what I have read on Cees DeGroot's pages and others, the 28, 35, 135 (in the latest compact version) and the 200 are their best efforts and my experience would seem to bear this out. Generally, except for color rendition, I have to admit I get pretty much the same results with these four lenses as I do with the Zeiss equivalents. By the way, the 200mm is much smaller and lighter than my Zeiss 200/3.5 Tele-Tessar.
I'd like to say that sharpness-wise there is a huge difference to my eye between them, having paid much more for the Zeiss units, but to me there just isn't in the shots I take. However, I get a much higher percentage of shots that make me smile and sort of get lost in the image from the Zeiss lenses and I really can't put my finger on a qualitative difference except that I simply prefer the colors.
In zooms I have the Yashica 28-85, the 35-70, a 35-105, a 75-150, a 75-200 and an 80-200.
The 28-85 is pretty sharp for a zoom, and running it head to head against my CZ 28-85 I find it to be much smaller and lighter, has only a little more distortion at 28 than the Zeiss unit, and costs $50 used rather than $500. It can flare a bit at the wide angle end as well and is a "cold" lens color-wise compared to the CZ (same shooting session, same light and subjects, same camera body, same film, lenses changed mid-roll in comparison shoots to eliminate variables.) My take on this one is that if you are trying to replace a couple of primes and quality really matters, take the CZ with you and put up with the weight. If you are just in need of a "walking around" zoom lens for a variety of shots in non-challenging light, or think your gear might be at risk in a hostile environment, put the Yashica unit on an inexpensive body and don't worry about it.
I have nothing to compare it to since I don't own the Zeiss equivalent, but the 35-105/3.5 Yashica I have is one neat little lens, emphasis on little. It has a very useful range of focal lengths and my shots with it have all been pretty sharp. I think I paid less than $50 for it on e-bay and have more than gotten my money's worth out of it. I understand that the Zeiss 35-105 is a fantastic lens but is HUGE and is not cheap.
Along the same lines, the little Yashica 35-70 seems like an okay lens and costs almost nothing. In practice however, I always seem to wish that the wide end was just a tad wider. Also this lens does not seem to have quite the build quality of the 35-105, and I have broken two in normal use (aperature problems). Again, I have no Zeiss equivalent to compare it to. I did have a couple of Zeiss 28-70's, which seemed sharper than this particular lens from Yashica and made pictures with colors I really liked. I let them go because I got the 28-85 Zeiss, and the 28-70 is a two-touch and I had no other two-touch zooms. For a light zoom, I probably should have kept one of them!
I've not used the 75-150 much except for informal portraits outdoors. I've always used it wide open and the results are pleasing. I've never gone any bigger than an 8x10 print from this lens.
The 75-200 seems only a so-so piece of glass, and a bit slow at f/4.5 maximum aperature. I've had a few of these and they didn't seem as sharp as the CZ 80-200 zoom. It's not really any smaller than the Yashica 80-200 which at f/4 provides a little brighter focusing. I like the 80-200 Yashica in that it gives very similar results to my CZ 80-200 f/4, again with the exception of what I perceive to be warmer tones in color shots featuring oranges, reds, and white skin. It's also a $50 lens on e-bay.
One thing you should keep in mind is that there have been several versions of some of the Yashica ML and MC lenses, normally getting better in later variants. This is addressed on some of the web pages, most of which link off of
www.cedegroot.com.
I know this is a long post, sort of subjective and maybe not what you wanted, but I hope some of my impressions help. The bottom line is that when I am shooting to produce something I will look at over and over again, and where color matters to me, I will take the Zeiss lenses every time and I am glad I own them. They are a joy to shoot with.
On the other hand, my twelve Yashica lenses listed above all sit in the same cardboard box ready for action. They are what I started with or used to establish which Zeiss focal lengths I really wanted, and all of them collectively cost me less than $500.00. They do their jobs very well, are frequently loaned out to others, take great B&W shots, and are trotted out for shooting high school football games, swim meets, and action pistol matches where flying bodies, flying water, and flying lead could all be a problem. If one of them bites the dust in the line of duty, I'm not going to lose a lot of sleep over it. Of course, other's opinions and results may vary.
Tom