The 5D Mark II is amazing in low light. I don't have a D700, or a D3, an A900, or a 1DS Mark III on hand to compare pixels to pixels. Nor do I care. I like experiential reviews that talk about actually using a camera, not comparing spec sheets, DxO rankings, MTF charts, or other quantifiable things which have nothing to do with taking photos.
I do have a 40D. And I'm very happy with the 40D. But the 5D Mark II is on another planet in low light.
I can shoot in ISO 6400 without batting an eye — it's like ISO 1600 in my 40D. It lets me shoot my slow 28-135mm IS lens around the house with no problem.
I'm serious, it blows me away every time I look at the files — the ones I post are down-rezzed to 800 pixels, so take that as you will. But given what I'm used to, I am blown away.
Zoomed in at 100%, there is noise present. But I'd be up on top of a 3-meter print with a loupe to see it in real life. Who cares? I'll bet at 11x14 I wouldn't notice it at all. And if these pictures aren't hanging on my wall, what are they, really?
I'm reading a lot of opinions lately about the 5D Mark II — most of them have been formed without having ever shot with it. Or even holding it in their hand. Trust me, as someone who has actually been using the camera lately, it'll help you make some amazing photos. With this camera in hand, your bad photos are your fault.
I plan to write up a detailed post after I have to bring it back. And I'm no Canon fanboy, despite the fact that I'm a Canon shooter; I could care less what sort of camera and lens collection everyone else has. All I can say right now, as a photographer, is that I feel lucky to live in a day and age where photos like the ones above are possible in such little light.
Oh, and the "black dots" controversy I'm seeing out there...haven't seen it in any of my shots after a week of shooting. I'll certainly address it if I actually see it. Take that problem with a grain of salt — if it's widespread, Canon will correct it. And lots of people seem to slag off the 5D Mark II's autofocus capability. I'd ask you all: have you tried it? Over the past week I've been using it, I feel like the AF works great. I think sometimes people get caught up in quantifiables, like that the 5D Mark II and original 5D both have the same number of autofocus points. It is what it is.