I must admit I am very excited about the 5D mark II and need to restrain myself from buying one straight away.
Don't get me wrong, it won't be a replacement for my M8. Most sensible people realize different cameras have different strengths. For me the M8 is a beautiful camera which is ideal for when you want a small but very good quality rangefinder. The lenses are way better than Canon lenses. Without seeking to rovoke a fight, I don't think the sensor is as good as Canon's existing one in the 5D, especially so far as noise is concerned (where I can take excellent photos with my 5D at 1500 iso, I don't like to even go up to 640 on the M8), and of course the Canon sensor is full frame.
Being a bit of an equipment junky I keep experimenting with different cameras. Currently I am enjoying using Mamiya medium format cameras. But with a good quality sensor, and now over 21MP, the 5D mark II has to be considered one of the most attrractive cameras out there. I have been getting into studio work recently, and it is another area I where my M8 would not be first choice - rather the 5D and sometimes the Mamiya RZ67 Pro II. If anything I fear the 5D mark II will replace my medium format cameras (which I will keep but use less) if I get it, not my M8. In the past I had thought the ideal combo for travel was my M8 and Mamiya 7. Now the 5D mark II will be very tempting especially given its versitility with high ISOs (the new 5D mark II promises to be good quality at 3200 and above).
I recently purchases a Leica 28mm R lens and adaptor for my 5D, and I have a 50mm R lens too, so I could of course use those with the 5D when the occassion demanded a better quality lens than some of the Canon lenses. If Zeiss bring out a range that will make the 5D mark II even more attractive. I think some of the Canon L lenses are very good, but when I compare them to Mamiya on a MF camera they do not stand up.
So whilst I don't regeret shelling out the large sum for an M8 at the beginning of this year, and some lenses to go with it, and whilst I have only brought my 5D back ou for some commercial work and experimenting in a studio and macro work, I would love to get my hands on the 5D mark II and I think it is bordering on the churlish to knock the camera. As someone who has invested in a few ks worth of Canon glass I am also delighted they may now have stolen back the mantle from Nikon for best semi-professional/serious enthusiast DSLR.
Nick