@Edwin: Sorry, hadn't read your link. I'll check it out.
Hmm, I'm not sure I buy all the reasoning in that article, given that it lacks any citations. His explanation of "Numpty" ISO is analog-only gain, vs. "Smart" ISO which is simply metadata, which would force digital enhancement in some post-processing tool. He claims Numpty is worthless, while Smart is useful. There is a HUGE difference between amplifying a signal directly, and enhancing it digitally. I would take analog gain any day over digitally enhancing. I had a debate a while back about the merits of choosing the lowest ISO vs. choosing the highest ISO when shooting with someone on a forum I participate in. The merits of those two choices aside, the debate resulted in some visual examples of analog amplification vs. digital amplification that I think might be enlightening:http://photo.stackexchange.com/a/6622/124http://photo.stackexchange.com/a/6712/124
Farther down in the Numpty thread, he explains some of his own tests, and his own conclusions about the 5DII. It sounds like the -1/3rd and standard ISO settings up to ISO 1600 are all what he calls "Bargain ISO", and he claims they are better than ISO 3200 given that they are "analog". I can't really tell the difference between Numpty and Bargain ISO from his description, given that both are amplified electronically...but one seems to enhance noise a lot while the other only a little.
Either way, that article discusses only "native" ISO levels + ISO 50, but does not discuss the H expansion levels (12,800, 25,600). Given that he claimed ISO 50 was "Smart ISO", I would be willing to bet that both H1 and H2 are also "Smart ISO", which according to that article is simply a metadata value and therefor 100% digital enhancement.