How come the 5D2 IQ was impeccable before the D800? I find it still impeccable today. :|it was not when it was released
it is not much better after all the firmware updates
it made a big splash being the first CHEAP full-frame and those who wanted or needed that were so enthused with this new toy they paid little regard to its IQ shortcomings because it offered IQ benefits and features previously unavailable.
Endlessly trumpeting on that basis suckered me in to buying one; my most regrettable Canon purchase.
not for lack of due diligence, but for lack of honest and clear information about the 5D2's weaknesses, which became clear enough after I used it for a while. .. and after others began posted about its less than ideal low ISO FPN issues.
If you still find it meets your needs you either lucked out with a good one or you don't mind crushing your blacks a few more levels than some of us.
edit adding DxOmark comment below:
and THIS is what annoys me about DxOmarks results. not just that they assign a vague overall score to a camera, based solely on measured sensor merits, but that they do not adequately disclose the testing criteria and data in a way that would allow the technically astute reader the opportunity to evaluate the data on their own. And, for the most part that I've found, neither do other sensor tech sites.
When FPN affects a sensor to the degree some of the Canon's (and some other mfr's products) were affected, it would have been very valuable to have a good idea whether the noise was random and acceptable or whether it was patterned and possibly objectionable to prospective purchasers.
DxOmark's data is useful but incomplete and that makes it much less useful than it would have been in my particular instance.