Some time ago I had the chance to put several older Leica lenses to the test, here are the results.
I have never actually used any of these lenses, I just had them for a couple of minutes to shoot a test series.
I only had one copy per lens so my results might not be representative for other lenses,
Leitz Super-Angulon-R 1:4/21
Typical performance for UWA lens of it’s era.
The center is very good from f/4, midframe is okayish and the corners are soft.
Stopped down to f/11 even the corners are sharp but not very much so. CA is controllled quite well.
My Canon nFD 2.8/20 is a better performer for a whole  lot less money, but there are many other legacy lenses in the 20/21mm range which have worse corners.
Test
Leitz Wetzlar Elmarit-R Â 1:2.8/28
A pretty unremarkable performance, my 20 bucks Minolta MD 2.8/28 is slightly better.
At f/2.8 the center is a lillte soft, the midframe is noticeably worse and the corners are pretty bad.
It takes f/11Â for the corners to sharpen up to very good levels.
Test
Leitz Summicron-R 1:2/50
This lens showed an interesting performance pattern.
At f/2 the center is only ok, Â my other lenses like the Minolta MC 1.4/50 or Canon nFD 1.4/50 are noticeably sharper in the center at f/2. What is remarkable is that the corners are the best I have seen in any legacy 50mm lens at f/2.
At f/2.8 the center sharpens up a lot to excellent levels.
By f/4 the lens shows very good sharpness from corner to corner which is a stop earlier than I would expect from my Canons or Minoltas.
At F/5.6 it is excellent from corner to corner.
I think this is an excellent landscape lens  but for general purposes I would prefer other lenses because of the under performing center at f/2. My analysis is based on the analysis of these infinity shot so my conclusion might be a different one if I would use it for actual shooting.
Test
Leitz Tele-Elmarit-M 1:2.8/90
This is a very small lens with very good performance in the center and midframe from f/2.8 with a hint of purple fringing. The corners at f/2.8 are soft.
At f/4 the center improves to excellent levels, the other regions don#t see much improvement.
By f/11 the corners have reached a good level.
I think this lens will work quite well as a very compact portrait lens but for serious landscape work I would leave it at home.
Test
Leitz APO-Telyt-R 1:3.4/180
Writing about a lens is always easier when there are no defects to describe.
The lens shows excellent sharpness from f/3.4 corner to corner with only the smallest hint of CA in the corners.
Test
Conclusion
Based on these results I don’t get all the fuss about Leica lenses.
Sure, all lenses had a very nice finish, nicer than that of other manufacturers .
But if I look at the performance it is pretty similar to that of other lenses of the same era. The only lens which stood out was the 3.4/180 APO but I would suspect that a Nikon 2.8/180 ED AI-S isn’t any worse.
Of course there are other very important aspects about lenses which I couldn’t experience like bokeh, color and contrast or handling etc. but I would be surprised if these lenses performed that much better in these aspects than other lenses.
From what I have read and seen the more modern lenses from the 90’s and later are in fact often better than the designs of other manufacturers, but that’s not the case here.