Personally, i do own the 17-40 but i also own the 24-70 II, and comparing both lenses at 24/35/40 at f8 and above, the 17-40 just doesn't keep up with the 24-70…even at f2.8, it's sharper than the 17-40 at f8.
i find the only real benefit is having the 17-23 range and almost all the time you can get away with that by moving back a little.
Before i had the 24-70, i used the 17-40 alot, not only for landscapes but also for street…but now, i've not touched it in a long time..
So if someone had the $ to burn for a 16-35, i would say why not just go for the 24-70II instead…it's definately going to be sharper and better..IF you can live without the UWA range..i'm personally finding i'm not missing the below 24mm range…i would rather trade that for a better overall lens