The biggest issue for this lens is onion bokeh.
Yep, it's still got something ugly goin' on in OOF hilites, but so does Canon's v1 lens and, to a lesser degree, so does Nikon's.
I'm not sure it would put much pressure on the Nikon version, its pretty much inferior accross the board. It does have lower CA's, but sharpness really is not close. Its not priced at much less either. I was interested in seeing how it compared to the nikon 24-70G for my D800, but it would be a downgrade.
In the case of the old canon lens, it might be a better lens, but the focus shift bothers me, as you stop down, focus moves backward. Doesn't do that on the Nikon version either.
Yes, have to give the nod to Nikon's current lens for IQ at the moment. The Tammy does pretty well tho, and might be a suitable compromise if you want to save a few hundred $. Too bad it's priced a little hight at the moment, maybe it'll come down a tad in a year.
And that focus shift issue is something that really dogged me when shooting with their first 18-270mm and that's really annoying. I was shooting some really nice close-ups of fruit on a tree, the backgrounds were terrific but when I got them back to the computer I found that on 2cm subjects the focus had shifted by nearly that much with only a few f-stops! All the shots were so OOF they were useless. Then the IS acted up and started causing
blurring. I got rid of it and picked up the Canon 18-200mm zoom instead; it works much more reliably and I still have it for a walk-around.
One would have to see to what extent the new 24-70 focus-shifts at "normal" shooting distances but it could be a deal-breaker.