I've had the 5D since it came out, and now the MkII. I also have a couple of M8's. I generally prefer the images I get with the M8's, but that's probably in large part because I take different types of pictures with the RF.
Looking purely at IQ, I find the M8 and 5D files approximately equivalent, with the 5D having about a stop better noise control at the top. The MkII is very, very slightly better but significantly better in this regard if you downsample properly into the range of the 5D or M8, and then has nearly 2 stops better noise performance.
Other image quality factors seem overall about equal; the Canons have a little better highlight info (can retrieve a bit more without crossovers) but the M8 files seem to respond better to underexposure and bringing up things in software. The Canon files seem muddier to me.
Part of the issue is that I can't really compare the two without their respective lenses, and though I have about 9 or 10 L lenses, only in the telephoto range are the Canon lenses comparable. So if I have images with a 90 degree angle of view taken with M8 or 5DII, the M8 files will almost always be better in IQ by a big margin because I've taken the M8 picture at any aperture with the WATE or 15 CV and the Canon picture with the 16-35 or 12-24 Sigma (my Sigma is very slightly better) at around f/8 or f/11 to bring the corners into some kind of shape. There goes the high ISO advantage. I'm anxiously waiting for the 21 ZE.
The Canon 35/1.4 is quite decent, but the 28 Summicron is better. Here, at least, the Canon is faster and is very usable for low light shots where the corners are not as important.
In very low light the Leica is still hard to beat. AF on the 5D and 5DII is not that great in very dark conditions, and I get way more keepers with the Leica, and as I mentioned before the Leica files respond nicely to boosting exposure in LR or PS.
So the Canon gets used for things like the 24 TSE (not really an outstanding lens; a friend promised me I'd get to try his 24 Nikon shift soon) in decent light, fisheye, macro and with lenses like the 70-200 IS and longer. Also with the 12-24 Sigma at 12 to 20mm as the 12 CV is difficult on the M8.
But, as others have said, the main thing is that the cameras are different and the image quality is really a secondary consideration as both are truly and in the best sense adequate.
I put together a vacation photo book (mostly for the benefit of my parents-in-law who are Ukrainian) at blurb recently:
http://www.blurb.com/bookstore/detail/507559
It's in their largest format, and while blurb is hardly the very highest quality, it's fairly demanding and typical of what you often need/want. I only used M8's on this little trip of 6 days with my wife, and it indicates that camera quality is certainly not lacking. With all files I had to throw away some resolution/quality to fit onto the page. I would not have done better with a Canon on this trip, and would not have been able to shoot many of the pictures I did. I would have been able to shoot others, but not these.
I will continue to use both, but for my personal stuff I generally prefer the Leica images.
Henning
