I have to agree with Berzerker on this one. I used my 5D at 1250 with no issues at all, and 1600 if I needed it. ISO 3200 was a step too far, though.
I didn't say that the quality above ISO 400 was not good, but it is certainly noticeably better at ISO 400 than ISO 800 (with slight pixel peeping) when printing. I said above ISO 400 was still fine for web use. I've shot at all native ISO's with the 5D, and love the film like results it gives on the higher ISOs, but I try hard to avoid it if I were printing. Since most of the time it is for web use, all native ISOs, if exposed properly, is fine.
Quite reasonable assessment. We all have different standards. I agree with you that proper exposure is an absolute necessity when using higher ISO settings. I found the 5D quite tolerable at 1600. I have no qualms about using my 6D at 25600, though I don't need to go that high often. I think there must be some voodoo going on in the 6D raw files. It is a bit odd that even at 25,600 I don't need any color noise reduction. All my crop cameras had atrocious color noise at 3200 and higher. The 6D, none. I shoot raw and use Lightroom 4.4, if that matters.
All in all, I don't miss much about the 5D. One thing I really liked that the 6D is lacking is the huge Ker-CHUNCK of a shutter sound. The 6D sounds a bit anemic, though quiet is good sometimes, too.