I know I will regret this comparison as there are bound to be posts from people who hate Leica and from those who want test chart shots but... I recently acquired a Leica R zoom 35-70 F4 - I bought to to use on my Leica New M body (when it shows up next year). The lens was $1000 NEW (well it is 12 years old but new). Although it is a mix of Leica design and glass and Kyocera construction Erwin Puts regards it as one of the best R series zooms (The F2.8 version of this lens is legendary but Leica was unable to manufacture it in any quantity) and suggests that it performs as well as the 50 Sumicron. So while I am waiting for my M I bought a Fotodiox "pro" adapter (without AF confirmation) and thought I would compare it to my Canon 24-70 F2.8 I. The Leica is obviously smaller, lighter and feels mach better made. I just took a few test shots in the garden and this is what I noticed. The Leica is sharper than the Canon at all apertures and focal lengths but the difference is not that great that it would matter to most photographers. Similarly the Leica shows less CA and the colours and contrast are slightly better. That said the differences in all of these are quite small and you really do need a side by side comparison to see the differences. The edges and corners of the Leica images are noticeably sharper that the Canon but this is likely due to the field curvature effects of the MkI 24-70. The Leica also shows no obvious distortion whereas the Canon shows slight distortion. On the other had the Canon is a stop faster, goes to 24mm and has AF. Probably the biggest difference between the two lenses is the transition from in focus to out of focus and the Bokeh. I have always found this difficult to show on the internet as you either end up with crops of the OOF highlights or a small image. Here are a few images - all taken from a tripod in live view (and Yes the Canon lens is micro-adjusted to this body) using my 5DII.