For me at least, there are three key M8 issues.
1. Price. For most of us, it's a lot of money.
2. We will lose a stop shooting at 35mm equivalent and wider. That can be worked around if the high ISO is reasonably good.
3. Digital is a different medium. Digital gives us convenience, can produce output faster, and relieves us of the time and trouble of scanning. Film has a different look, and is more forgiving in the exposure and dynamic range departments. Pick your poison.
None of these issues are deal-breakers for me, with the possible exception of the price.
The M8 appears to have good image quality on at least par with the DMR. People I know and respect who have shot Canon 5D or 20D plus the M8 seem to like the M8. The M8 may lead to the purchase of one additional wider lens, and most of what we already have will work fine.
I tend to photograph mostly with a 35 and a 50. The 50 is my favorite focal length. Trust me, the advantages of a rangefinder apply very much to normal lenses, not just wides. With the M8, my 35mm lenses will become, effectively, almost-50mm lenses. My 50 will become a little "longer." I can experiment, and I think it will be OK. If I want a fast 35mm equivalent, I can pick up a VC 28 Ultron. Leica and VC both have 28mm lenses, and there are other old lenses to try as well.
And my 90 Summicron will become and effective 120mm f/2. Can't wait to try that in a concert hall or theater.
If you're really into very wide lenses, then the crop factor might be an issue. If you are absolutely into a specific focal length other than 50mm, there might be some adjustments. You either deal with it or you don't.
All the quibbles pale when I consider that the M8 is a digital rangefinder that looks, feels and handles much like my film M cameras, and takes Leica lenses. It will allow me to shoot digital the way I like to shoot film--with rangefinder viewing and focusing, using a small, light, unobtrusive camera and fast, high-quality optics. The M8 looks like a winner.
Basically, the M8 bumps each lens up one "notch" in equivalent magnification. My opinion is that the minor focal length differences won't matter. Yes, your 28 will become a 37mm lens instead of a 35. But it's just not all that significant in practical shooting. You'll get used to it in a day.
And if price is an object, there is the R-D1 alternative, albeit with some other issues and risks.
--Peter