I just did lots of research about this in the past couple of weeks so I thought I'd post my 2 cents.
Which 35mm lens should you get? It depends on the the look you want and your budget. We are fortunate in that there are so many excellent lens choices available nowadays. Leica / Cosina / Zeiss, etc. There are also some great legacy lenses from Canon, or even the FSU Jupiter-12, if you can find a good one.
Personally, I am not a huge fan of the CV 35/1.4 but other people like it very much. I am anticipating the arrival of a C-Biogon (to replace my CV 35/1.4) in the mail very soon. I like the Zeiss 'look', and the fast speed of the CV 35/1.4 wasn't very important for me. The CV 35/1.4 is nicely built and is a good value, but the images are just not to my taste.
The Zeiss C-Biogon 35/2.8 is insanely sharp and has virtually zero distortion. I have yet to read a bad review about this lens. I think Zeiss has created a real winner with this product.
I have also read good things about the Zeiss 35/2.0 ... when shooting film. Supposedly the Zeiss 35/2.0 is prone to chromatic aberration when shot wide open on digital bodies like the M8 or M9 -- but since you're using a M6 body, you would be fine. Again, the Zeiss 35/2.0 seems to receive universal praise. Yes, it has some small imperfections, but for the price, it's almost a 'Summicron killer'. Many people have reviewed this lens online and given it high marks. Ken Rockwell loved this lens so much he slathered on the hyperbole and praise with more gusto than usual. I'm surprised he didn't adopt it into his family and proclaim it his 3rd Rockwell child!
The CV 35/2.5 skopar seems to be universally loved, it's a cheap but excellent lens and probably destined to become a classic. Very tiny and it produces excellent images. Razor sharp in the middle with a bit of vignetting and distortion in the corners when wide-open, then a stellar performer when stopped down. Lots of love happening with this lens.
Sean Reid and Mr Putts both love the Summarit 35/2.5, but if possible I would buy this particular lens on the used market ... if you're going to spend that much money on a lens brand-new, then you are heading into used Summicron territory anyway. The logic of getting a new Summarit starts to get muddy in that price range. In the Summarit's defense, I have read many reviews that state the performance of the Summarit + Summicron at around F2.5 is virtually identical. That's high praise, indeed.
The CV 35/1.2 (version I) is a unique lens, many people love it, but some people don't. It tends to be a fave of people who do a lot of low-light shooting ... it's a medium-contrast bokeh king. Much cheaper (in price) than the comparable Leica product, but well made and quite versatile. But it's a largish lens, and it has its own 'look' that you either love or you don't. I've heard it has a tendency towards veiling flare in bright sunlight, but I'm only repeating what I've read online. I think it's a good lens, but only if you're into low-light shots, or you're into using ND filters for daytime and you love razor-thin DOF.
Cosina Voigtlander will be releasing the Version II of the CV 35/1.2 very shortly ... it's supposed to come out in August. I am waiting for the early-adopters to try it out and then I'll read the reviews to see what it's like. Supposedly they changed the glass formula a bit due to supplier issues, and the Version II will be a slightly different (improved?) lens. We'll have to see.
I could go on and on ... but I'll shut up for now. Like I mentioned before, there are many many good choices out there, it just depends on your budget and shooting style. In this matter, the search engine is your friend (the Google one, not the RFF one! LOL!) ... there's lots of good info out there.
