The one guy who says he switched to Nikon for their 24-70....uh....ok...? (*scratches head*)I didn't switch to Nikon for their 24-70 specifically, if I had to name one thing it would be the AF of the D700 vs the AF of the 5DII(I did this over a year ago when the 5DIII was still $3500 and new 5DII's were around $2K.
I had an old 5DC with a 24-105, but I missed focus on half my shots and just constantly was disappointed when conditions were not perfect. I also like the pop up flash on the D700, it saves me from having to lug around a separate flash 90% of the time.
I traded my 5DC and lens for the 24-70 2.8 Nikon pretty much, then paid 1400 more for a used but basically mint D700. I think I made a huge upgrade from the 5DC and 24-105 without having to pay 5000 dollars. I hardly ever hear anyone say anything bad about the Nikon 24-70, and I've personally never used a better lens. The Canon might be sharper, but at this point, I wouldn't trade my Nikon 24-70 for anything less than a FF mirrorless system that is much smaller and lighter.
I travel a lot and am sick of DSLR's. I'm now looking to only use my EOS-M and hopefully something a little(or lot) better but with a similar size in the future.