When compared side by side, the summicron excel in slightly higher contrast and distorsion free. However, the lux gives more shadows penetration in dark environments, but slightly flares more in direct sun shines . Cover on lens hood falls off too easily. New lens works extremely well with new film emulsions. Won't leave home without it.
Image quality is beyond words, size is tiny.
It's not cheap, but once you see your results, you'll never want to part with the lense. With 6-10 years experience in People photography. The new 28 f2.8 and the 50 f2 are close seconds. Too slow and expensive.