The zoom ring is pretty smooth out of the box with some resistance. Overtime, this resistance might weaken a bit, but I have not had the lens long enough to tell for sure. My Nikon 24-70mm was fairly good in the beginning and now the zoom ring is too smooth and somewhat loose – a result of heavy field use and abuse (I will be sending it to Nikon for tuning soon). The zoom and focus rings are reversed like on some Nikon lenses like Nikkor 24-120mm f/4 VR and the zoom ring is not as wide, which I am fine with.
While inspecting the lens, I found out that zooming in to 70mm causes the rear element to go deep inside the lens. While some movement of the rear element is quite normal on zoom and prime lenses, I found this one to be a little extreme. At 70mm, a lot of the lens guts are exposed, as seen in the below image:
Read more: http://photographylife.com/reviews/tamron-24-70mm-f2-8/2#ixzz2yCTKhZ25