Abram, I have tried lots of the different Leitz 50mm lenses, made over time and have used them mostly on digital M bodies, my favorites of those lenses also having seen some use on film.
I once wrote a blog post about the 50 Summilux pre ASPH being
"the most beautiful Summicron".
I have used the 50/2 Summar, Summitar, Summicron v1 (collapsible), Summicron v4 (Canada tab) and v5.
I do not very much like the modern Summicron optics (starting from v4 and still used in v5). This lens always had me feel something in the image is missing.
It is a sharp, contrasty, modern lens, which will not upset anybody - a bit like a Volkswagen Passat - it does what it is supposed to be doing and nothing more.
I am not a Passat person. I rather drive a more exciting automobile.
The pre ASPH Summilux has some flaws which the users who really like it over look - or if convenient have a second lens for - like a Summicron ;-)
The pre ASPH Summilux will never get as uniformly sharp over the frame as the Summicron (v4/ v5) at any aperture. I compared here a late Canadian 3Mio v2 pre ASPH, a silver chrome E46 and a black paint E46 against the two modern Summicron samples I owned. They all behave very similar.
You can try to stop down the Summilux to f5.6 or f8 and it's center will always be a bit sharper and better defined than the outer perimeter.
If critical architecture or landscape with large enlargements is your thing, the Summicron will lead to better results in sharpness.
Personally for me though sharpness is indeed important but not a primary characteristic of a lens I seek after - many lenses I truly enjoy are what people call "soft" or mediocre, but these do deliver something or other I like about a lens.
To me, the pre ASPH Summilux delivers a balance of just about perfect contrast, beautiful tones, a truly wonderful gentle rolloff from sharp focus to smooth out of focus backgrounds, that from the modern lenses (1980 and later) it is my preferred 50mm.
I would take ANY pre ASPH Summilux over a modern Summicron any day for that reason.
But then I do not photograph architecture or landscapes and certainly am not nitpicking on corner sharpness …
Here are some sets that may be possible to illustrate a little of the character between the two:
50/2 Summicron v5 silver chrome
(I should have never sold that lens, but it helped financing a medium format camera I really wanted more than keeping another Summicron):
portrait - gentleman in HK by
teknopunk.com, on Flickr
The Summicron is really sharp - always.
50/1.4 Summilux E46 black paint:
candid portrait - old man by
teknopunk.com, on Flickr
Look at the skin tones - this is very typical for the Summilux. Somehow it just takes the edge off the harsh look of the Summicron and seems to just produce nicer tones. You can see in the background that I probably stopped down to ~f2 here, one can therefore compare to the Summicron.
50/2 Summicron v4 (Canadian tab):
portrait - black frames by
teknopunk.com, on Flickr
This is my perfect example for the look of the Summicron I do not like.
50/1.4 Summilux E46 black paint:
portrait - gentleman with sunglasses by
teknopunk.com, on Flickr
This was shot in very similar lighting condition. The portrait of the young woman above done in morning light, chignon straight in her face, the portrait of the elderly gentleman in evening light at a very similar angle.
The Summilux shot has been done stopped down a little - maybe f1.8 - f2. The Summicron shot with the young woman above was at f2 and I used a 022 yellow filter (which brightened her skin tone)
I experiment always with new lenses how much I tend to filter them and generally keep a certain strength filter for certain lenses. With the Summicron I tried during that time to use yellow filters and didn't like the outcome much.
On my E46 Summilux, I basically keep a B+W 022 yellow filter on the lens at all times during day light - it is the perfect balance of tones and contrast to my eye with this lens and I hardly do any post processing on the resulting digital files.
With the Summicron I always had to make a great effort to get the look, as I want.
Here is one shot with the Canadian v4 Summicron I liked (I think it is much more the character of my subject than the lens' character in this case):
Leipzig portrait - gentleman with glasses beard and beret by
teknopunk.com, on Flickr
Here is an example of the tonality, I like to achieve in a portrait. It is not easy to get this look, some lenses simply seem to deny my these tones.
The pre ASPH Summilux is among the very few lenses that really produce this very look (contrast, tones, skin tones):
portrait - chinese by
teknopunk.com, on Flickr
I think it is this character of the pre ASPH Summilux why I love this lens so much and why I can completely over look it's negative characteristics (which in effect do no matter much to my photography, opposed to maybe a serious landscape photographer).
Personally I do love people. I do love the moment when one in a thousand portrait subjects seem to crack a little and reveal character, allowing a glimpse beyond the mask we wear in public everyday (I do 99% of my portraits with total strangers on the first few seconds upon seeing them the first time).
The pre ASPH Summilux somehow allows me a better chance to get there more often.
Just for fun, here is a street portrait of the very earliest Leitz 50/2 lens - made with a very nice copy of an uncoated 5cm Summar:
portrait - two women - fashion by
teknopunk.com, on Flickr
This is an example why I couldn't ever have only one single lens per focal length - these lenses all have so extraordinarily different character, that one really has to have them all ;-)
… or at least die trying to have had tried them all until boxed up and buried.