Ive owned most of the ultra wide zooms and primes over the years. I started with the Tokina AF 20-35mm f3.5/4.5 Then the ATX 20-35 f2.8 followed by Nikon 20-35mm 2.8, Nikon AFD 18-35mm 3.5/4.5,Nikon AFS17-35mm 2.8, AFS14-24 2.8, AFS VR16-35 f4.0, Sigma 17-35mm 2.8/4,Sigma 15-30mm 3.5/4.5 and then there are DX fit ones,
TBH I can't believe i struck lucky ith every one i purchased but none of them were dogs!
The one i should have kept or not traded ?
Tokina Af20-35mm 3.5/4.5 sharp contrasty very little distortion and better than the 2.8 Version !
Sigma AF 17-35mm f2.8/4 still have some great shots to process taken with this in the lake district and Wales.
Nikon AFD 18-35mm 3.5/4.5 as sharp as ant of the other Nikon UWA takes filters if you want to use and light weight.
today i have for
DX Tokina 10-17mm 3.5/4.5 F.F. fisheye to 17mm, Canon 10-22mm 3.5/4.5 APSc Fujinon XF10-24 f4 ois
FX Nikon AFS-VR 16-35mm f4 and the Sigma 15-35mm x 2 one for my Canon FF
Just remember the older lenses like Nikon AFS 17-35mm f2.8 and the AFS 28-70mm f2.8 are becoming harder to get serviceable parts for as they are discontinued lenses the same applies tp the AFS80-200mm f2.8.which is a pity really.
So I would choose carefully and go try a couple out the photography show isn't that far away if you can get there?
If your going to go the used/s/h route make sure that you have the option to return if you not satisfied with it.
Any thing that has a inbuilt motor check its not squeaking or making any strange noises and zoom action is smooth and not jerky or stiff, and it goes without saying carefully check the optics for fungi and marks/scratches along with filter thread or other damage
