I have been using the 35-70/4 for several years. It's an extremely sharp lens due to its ASPH lens surface. In my own experience, I felt it is sharper than both Summicron 35 and 50 at their respective F stops. But it has different image characteristics compared with Summicrons. In the beginning, I felt the 35-70/4 was 'too' sharp and somewhat lack of smoothness that I was familiar with the Summicron 35. But I've grown to like the lens more over time. It's become the standard lens on my R8 when I travel with Leica R system. If you can live with the F4 and limited range, it's one of the best R lenses available, irregardless of the price, IMHO.
For me, the biggest problem of whether to keep my R lenses is the future of R system.
Kind regards,