This was my very first Leica lens, it was the lens that started my Leica obsession. Funnily enough I bought it thinking that it would be the last lens I would ever need! It did exactly the opposite: once I owned this lens I needed to try the next Leica lens and then the next, probably until I have tried them all... basically it started a very expensive hobby.
Previously, I had bought the 50mm Zeiss Sonar M, but I wasn't always happy with it. Although it has an amazing and unique characteristic, it's a little soft. This started to frustrate me and so I went looking for another 50mm. The 50mm Summicron, on the other hand, is probably one of the sharpest lenses ever made! On top of this, it's a true Leica lens with beautiful Leica character and colours. It was a little expensive for me, but from the reviews I was reading it seemed like the only Leica lens I would ever need. So, I decided to save up a little and get it for my Sony a7. After all, if I was going to use it for the rest of my life it wouldn't really matter if it cost me a little extra.
Buying this lens eventually resulted in me selling my Sony and buying the Leica M3. As I explain in my M3 review, the M3 led to the M6 and me choosing 35mm over 50mm. I sold the 50mm to pay for (part of) the M6 and 35mm Summicron v3, both of which I still own today!
This is an amazing lens. It's sharp, small for a 50mm f/2, lightweight, and just looks gorgeous. It also has a focussing tab unlike the newer v5 model, which lets you focus much faster. Leica hasn't changed the optics for it's 50mm Summicron since 1979 because it doesn't need to. The focus is also smooth and aperture clicks very satisfying.
However, this is a very clinical lens - just like all 50mm Summicron's that came after it. Although it definitely has Leica character, it doesn't have that much character in my opinion because it renders everything in such a sharp and accurate manner. 50mm can also be quite a limiting focal length, and can only really bring part of a scene into focus. Sometimes this helps you select only the most interesting part of a scene, and improves your picture... but other times (more often, in my experience) 50mm is too narrow to really tell the story of what is happening.
Overall, this is a great lens, I would advise it to anyone. That being said, I took almost no pictures with this lens, especially given the amount of time that I owned it. This is mainly because I found it to extend outwards too much when mounted on the M3 (it was too long). It's size bugged me, it didn't feel right for me on an M body, and it didn't fit in my coat pocket with the lens mounted. I found it too unpractical to carry around for casual street-photography. That - in part - is also why I was so happy with the 35mm Summicron v3, it's so small that I started to feel like I could take my camera anywhere with me (and as a result, I did). It easily fits in a coat pocket, which is how I usually carried my camera at the time.
This is also why I personally prefer the 50mm f/2.5 Summarit M, it's much smaller than the Summicron, and it can be carried as an extra lens in my inner coat-pocket. Also, I find it has more character, feels just as nice to use (except maybe that the aperture is a little less satisfying) and is basically as sharp. Even now that my photography is a lot less casual - I still much prefer to have a lens with a smaller profile like the Summarit, if I can.
I really wanted to like this lens, I wanted it to be the last lens I ever bought. But it didn't have that effect on me, there was no spark or click. Technically in terms of quality, sharpness and performance this is an amazing lens, that was obvious. But technical performance was not why I had spent extra money for a Leica lens. These days every major camera manufacturer, including the apple with the iPhone, can make technically amazing lenses. To me, the Leica M system is all about getting away from this trend, and owning a lens that has a such a distinctive and unique character that it just blows you away - not just because it is so sharp.
There just isn't that much to say about this lens, and maybe that says enough. In many ways it's one of the most basic, all-round, 'Leica' Leica lenses. It just feels like it's 'finished', which is probably why Leica never bothered to make any changes to it's optical design in later versions. It really is a lens you could own for the rest of your life, and never use another. But theres also not very much that's 'special' about this lens, which is why if I had to sum up how I see this lens in one word, it would probably be... boring.