I have got into this M42 lark too.
Have ebayed a Carl Zeiss 135, a Super Takumar 50mm1.4 and a set of macro bellows for it. Also have a Zenitar fisheye, but its a canon mount version but I think it originated as M42?
I received the CZ yday and wow. Sharp wide open, although 3.5 is quite slow for me. Manual focusing with it on a 350D is also very easy, compared to the fish eye, that thing is a nightmare as there is so much in the screen it is impossible unless set to infinity....Any help with that respect?
Am eagerly waiting for the Takumar.
I have Eos 28mm1.8(walkaround main lens) and Eos 85mm1.8(portrait) as my 2 workhorse lens, but it is nice to fill in the gaps in my lineup very inexpensively. I was going to buy a Canon 50mm1.4 to replace my cheap ol 1.8, but never did because I would use other lens as Its too close to the 28 or 85, i can just step back or forward a bit.
So the Takumar shall be a nice way too have my 50mm1.4 a lot cheaper.
Also have always wanted macro capabilities, so can use my Takumar and macro extensions ( £2
and now i also have a macro set up....very cheap.
I also wanted the Jupiter 85mm, but dont think I want to reapeat a focal length as it will never be used.....but very good bokeh? better than the Canon?