There's just one problem for Tamron's new lens: Nikon also has a new lens - the Nikkor 55-300 VR, which not only covers a greater range, but also costs the same or less (even after the $50 rebate on Tamron's 70-300 VC). I don't know about the optical quality of either lens but I would guess the Nikon has the same or better quality.
Even if Tamron drops the price on the 70-300 VC, the 55-70 range is such a useful range that it is hard to imagine getting the 70-300 VC.
For similar reasons, I think the 55-300 VR may seriously cannibalize sales of the well-regarded Nikkor 70-300 VR unless the 55-300 VR has terrible quality, which I doubt.
If the 55-300 VR turns out to be a decent if not good lens, it appears the Tamron 70-300 VC and Nikkor 70-300 VR would be useful only for full-frame cameras.