I know that I lucked out picking it up very cheaply but, close to J-12 price once you subtract out the value of the included M adapter-
The Canon 35/2 is small, fast, and VERY sharp.
The major problem with the J-12 is that rear element. A lot of cameras just can't take it, either in Contax mount or LTM. You cannot put it on a Contax IIIa or IIa. It fits on "many" Nikon RF's, but some filing might be required. And on LTM and M-Mount cameras, you need to be careful of light baffles and meters. A Meter stalk can get "hit", and some TTL meters get their view obstructed.
> I used on J-12 recently on a Leotax.
With all that stated, put a J-12 on a Barnack style camera and you are set. It would go right onto my Leotax D-IV. But then I would have to take the Simlar 5cm F1.5 off of it...
I end up shooting with the Canon 35/2.8 and now 35/2 for a compact setup. The 35/2.8 is "just compact", and much sharper than the Nikkor 3.5cm F3.5. Another low-priced 35 to look at.
On J-12's scratching Nikons
> it is with the rim.
> it will put a semi circular clean scratch on the trim plate of your Nikon.
That is the issue. Now, MY "Beater" S2 picked up very cheap has every scratch possible on it. EXCEPT the J-12 rim scratch, I polished it down. Just lay Sand Paper flat and use circular motions. Take just a little bit off of it. Consider it "Conservation of Inconvenience" or "Price vs Elbow Grease". AND if you really want it perfect for a Nikon, take off the rear module, unscrew the front module to reveal the Shim. Increase it's thichness by ~0.4mm or so.
(Okay- so Optics forum, I've generalized this to 35's for street (and other) photography)