Above are two versions of the same file. One from raw conversion and one from the OOC jpg. Considering the pixel density, I don't think this is bad considering it's ISO 6400, which is the cameras highest non-extended option for low light. There is ISO 12800, but that's one of those underexpose 1 stop and lighten up the image alternatives, so I've not shown an example of that.
Below is another ISO 6400 file comparison, but the single image below that is what highly populated sensors have up their sleeve. They can be significantly downsized, still produce a decent sized file and because of that downsizing the file gets a lot less noisy. So the second (lower) 100% blow up is with the files downsized to 12MP, the same size as the Sony A7s. As you can see it's a lot cleaner and in fact is very good indeed.
The other thing to notice here is that the images at these high ISO settings are not over noise reduced, which is good to see. I have to say, I've found this somewhat surprising given Canons record of dealing with this. However the files are refreshingly sharp and clean. The point about sensors and high ISO's is that things move on. They get more efficient at 'capturing light' and every time the pixel size gets increased, the sensor manufacturers make sure that there is an improvement that can be clearly seen. That is very much the case here.
One of the suggested uses for this camera has been wedding photography and I see the case for that. I'd have certainly been very happy with the results from this camera when I was shooting weddings. To be honest I never shot anything at more than ISO 1600 anyway when I was using Canon DSLR's for that, choosing to use one of Canons excellent flash guns, usually bounced of a ceiling or a reflector. But if you need decent low light performance in anything resembling a 'normal' shooting situation, then the 5Ds will turn in a decent performance. Black cat down a coal mine? Well you may have have to consider other alternatives for that.