Why woudl you want to do that NOW?
Canon's 24-70 II and 70-200 II L DESTROYED Nikon's counterparts, at least based on 5 professional reviews.
5d3 beats D800 and 1DX beats D4, based on 3 professional reviews!
All depends on what you're doing. If you're mainly doing landscapes then the above is definitely not true. The Low ISO performance of the D600 and D800 are superior...
Then you have to take into account price. A D600 and Nikon 24-70 is significantly cheaper than a 5DMkIII and 24-70 II (about $2000 from reputable dealers). If this is just a hobby... that can be significant.
For instance... there was my situation. I had a 7D and a 70-200 f/4L IS. I also had a bunch of smaller gear and older gear (XSi, old plastic lenses) and a broken 17-55 f/2.8. Let's analyze.
If I were going to move up to a 5DMkIII I would be selling my 7D. That ended up netting me $1200. The rest of my gear wouldn't give me that much cash (maybe ~$800). So now I have $2k to spend.
If I go get a 5DMkIII body from Amazon (not that I would get it there, just convenient to talk about) it costs me $3,200... so I'm out of pocket $1,200. That's ok, because I've been saving for a camera upgrade and have some cash. But now what? Oh, I need to get a 24-70 II as well (it's my preferred range and a damn fine lens). So now I'm out of pocket another $2100... for a total out of pocket cost of $3200.
The nice thing about this is that I still get to use my 70-200 f/4. The downside is that I'm out of pocket $3200.... which would make my wife fairly unhappy...
Now let's look at switching. I would have the $2k I had before... plus I could sell my 70-200 f/4 (which I got $1000 for) so now I have $3000. A D600 and 24-70 together cost $4000... so I'm out of pocket $1000.
This has the very real drawback of not having my 70-200 f/4 afterward... but I don't need it right away (again, mainly landscapes and travel photogging). I can save up for it and snag it later (Nikon just came out with one for $1400)
So for $1000 out of pocket I have a D600 which is awesome for landscape photography and a really great 24-70 (can't say it's better or worse than a 24-70 II myself... but it is really good either way). That is much easier for my wife to deal with... and I'm getting excellent IQ (orders of magnitude better than the 7D... but that's expected. Can't say how much better than a 5DMkIII).
Yes, I could have gone for a 5DMkII... but I would have sorely missed having good AF (I don't _only_ shoot landscapes!) and still wouldn't have the low ISO DR and IQ that the D600 does. Nor would I have access to the excellent Nikon 14-24 (Canon ultra-wides are really not great - I bought a 16-35 this year... and went through two copies before returning it all together because of lack of resolution at the edges... even on my 7D!)
Personally, if Canon would have come out with a serviceable 6D I would have stayed. But there were just too many features missing for the price (might still be a fine camera, but I feel like I got more for my money from a D600).
Basically, this whole thing is just trying to let some of you see that there are reasons to switch. The world is not black & white. There are many people in all sorts of different situations and with different needs and in different financial circumstances. It's not as easy as "X reviews better than Y!"... there are more variables...