Of course, MTF's are
the indicator of quality. However, I haven't noted any comparisons of the Canon V Leica glassware. If you have please point us in the direction.
That technical issue aside, we don't always require/desire maximum quality from our lenses. I love the old soft Leica 'look', whatever it is. I see it in the Canon 135L but I can't reproduce it in my Leica M9.
Never mind I'll just continue to plough my lonely furrow on TP with my rich mans toy. Going out to play now.
MTF is the measure of sharpness, but there are other things - vignetting, distortion etc, and flare is often overlooked but can have a huge impact on contrast.
There are some Leica MTF graphs linked above. Here
http://en.leica-camera.com/photography/m_system/lenses/2183.html click on the Know-How button.
They are only an MTF snapshot and you have to be careful exactly what you're looking at. The Leica graphs are at 5, 10, 20 and 40lppmm, whereas Canon (Nikon and Sigma too I think) present only 10 and 30lppmm.
That's one thing, and also those values are an old standard TBH, relating more to film that digital. For example, the sensor on a 7D with 18mp is capable (in theory) of resolving 116lppmm

but is overlaid with an AA filter at an unknown level.
Radiohead's point about the Leica M9 having a very mild AA filter is an interesting one and the M9 should therefore be able to get closer to its theoretical maximum than other cameras with a stronger cut filter, but that is one thing that manufacturers never quantify (because it makes a nonsense of their marketing rhetoric about high pixel counts).
So at the end of the day, all the manufacturer's MTF graphs are only an indicator of lens potential in actual use. You have to read between the lines (haha

). IMHO what is significant about these new Canon lenses is that they are now paying much more attention to very high resolution levels, both to better exploit the potential of the 30-40mp sensors that are around the corner, and also to make extenders/teleconverters an increasingly viable option.