serious no one should expect it to e as good as the nikon ($2k++ lens) unless the tokina is also selling at $2k+. direction comparison should be the sigma 18-50 or tamron 17-50.
The Tamron 17-50 f/2.8 offers optical performance that is as good as the Nikon 17-55 f/2.8 and Canon 17-55 f/2.8 IS. But it does not have the built of the Nikon or the IS of the Canon. AF is also noisier and slightly slower.
But the Tamron easily trumps the Sigma equivalent unless Sigma made some improvements with their new macro version.