Let's talk seriously about what Ted calls "IQ," image quality.
Firstly, subjective image quality depends more than anything else on the light and the subject matter when the exposure is made. Good cross-lighting and a high-frequency subject like a face behind a window screen will give much better apparent image quality than flat lighting and a subject composed mostly of simple, more or less rounded objects.
Secondly, throwing images onto a computer screen at 97 PPI or so isn't going to tell you anything about IQ. The computer screen just isn't capable of showing more than the coarsest differences in IQ.
Thirdly, the ultimate quality of a digital image file is going to depend on both in-camera processing and post processing in Photoshop or its equivalent (actually there isn't a Photoshop equivalent, but you get my point). If you shoot raw, which is what everyone should be shooting, most of a digital image file's quality is going to depend on your original exposure being right on the money, and your ability properly to adjust the white point, brightness range and sharpness of the file. Post processing isn't a trivial skill. It takes practice.
Fourthly, what matters in the end is the quality of the print rather than the quality of the intermediate file. Assuming you have a really well crafted digital image file, the quality of the final product, the print, is going to depend on the quality of your printer, proper selection of ink and paper, and the quality of the color profile you're using to match printer, ink and paper. It's the IQ of the print that really matters. Everything else is academic.
Leica chose to use a UVIR filter over the sensor that's about half the thickness of most sensor filters. The plus side is that the thin filter can give you extra image quality. The minus side, of course, is the UV contamination problem. So, all other things being equal, the image coming out of the M8 potentially can be better than the image coming from, say, a Nikon. But "potentially" is the operative word. In most cases all other things aren't equal, and in most cases the difference isn't going to be visible in the final print unless all steps in the process are done equally well.
My Nikons produce excellent image quality, as does my R-D1. Whether or not the M8 produces better IQ depends on a raft of factors besides the M8 sensor.
Finally, it's worth thinking about this quote from Cartier-Bresson:
"I am constantly amused by the notion that some people have about photographic technique - a notion which reveals itself in an insatiable craving for sharpness of images. Is this the passion of an obsession? Or do these people hope, by this trompe l'oeil technique, to get to closer grips with reality? In either case, they are just as far away from the real problem as those of that other generation which used to endow all its photographic anecdotes with an intentional unsharpness such as was deemed to be 'artistic.'"