After about a year with the D800E and a short period with a couple of D800's (one IR720nm and the other full spectrum), I think the D800 and E are all extremely mature platforms, and the IQ is very good.
I used Canon for the past 12 years, and just ventured into Nikon with the D800E. A few things worked better for me....and not necessarily just IQ. I liked the ergonomics a lot better, much more tactile. I liked the auto ISO implementation a lot better. Most importantly, I felt some of the glass in the 14-100mm range was stellar. Beyond this, I think it is a close call either way, with the exception of the Canon 135/F2 which really does not have a similar play in Nikon. I am hoping the new Zeiss will fill that gap, albeit in MF.
Minor quibbles -- green cast on the LCD, should have been addressed a long time ago. LV, marginally better on the 5DIII. Video, again, perhaps a little better for critical work, but not my thing.
So, I am very happy with the D800's and D800E. I also like the fact that Nikon has a more gradual semi-pro to pro line. I had the 5DII and IDSIII at one point, and these were two different animals. What a waste of a great sensor in the 5DII with the AF system that it had.
I still miss my 85/1.2 (the 85/1.4 Nikkor is still a stellar lens), the MP-E65, and then135F2. Otherwise, no complaints. The 24/25/85 1.4G are all very good, and I do feel I gained some IQ in these lenses. The 14-24 is fantastic as a single UWA. The 200F2 is in someways better ergonomically than my previous Canon 200F2. The recent addition of a Coastal Optics 60mm Macro has made my Nikon lens lineup even more exciting.
Sorry for the ramble....as for IQ, I still still reach for my other kit when I have the time to invest on being creative.