DxOmark tested the Leica SL 35mm f/2.0 lens and concluded:
Many professionals and high-end amateurs consider the 35mm focal length to be an alternative to the 50mm, and as a result, require models with similar or even better optical quality. However, as lens sizes have increased over the years to offer superior image quality, there remains a need for smaller, lighter lenses that offer similar image quality.
Leica’s APO-Summicron-SL series appears to offer just that, and its smaller, lighter f/2 lenses are as well or better corrected than “standard” f/1.4 versions. With such an exotic optical construction, then, the Leica APO-Summicron-SL 35mm F2 ASPH has many attractive qualities, including good sharpness starting at f/2, low LCA, and almost no distortion. It’s also almost as bright as some f/1.8 lenses. All of this comes at price, though, and the $4595 ticket isn’t for the faint-hearted.