At first, I thought IS was kind of silly until I thought of it more. For me at least, it is rare that I use a tripod unless I specifically need low ISO, long night exposures. Otherwise, I generally will just hand-hold at higher ISO which does not bother me. However, if we gain 2-3 stops of hand-holdability, along with a high resolution wide open performance… This can be *really* nice.
For example, instead of using something like a 16-35mm f/2.8 II at f/8, 1/50 at ISO 3200 to get a specific shot without a tripod… You can use this new 16-35 f/4 IS at f/4 and still get better sharpness across the frame than the 16-35 II at f/8, and reduce the ISO to 800 by using the larger aperture alone. Add in 2-3 stops of IS, you are now at ISO 200 or even 100. Your end result will be a higher resolution image (from both lower ISO and lens performance, along with better dynamic range).
If you require the same depth of field, you still will reduce down to ISO 800 or 400 with the IS alone over the 16-35 II at the same aperture. Which again, will increase that dynamic range and lower noise.
I don’t use UWA often to stop motion myself, that will generally be left up to faster primes or my 24-70 II or other lenses. I think for many, UWA zooms are for scenic landscape type shooting. We also will no longer have to stop down to f/8, f/11 or even smaller to get high sharpness to the edges/corners… So in one sense, this lens is *much* faster than the existing 16-35 II
When I use my 17-40, my “default” aperture is f/7.1 to sharpen the edges up enough. I will use f/5.6 or larger, but it depends. Having a solid f/4 performance better than the current options stopped down to f/11 is just very appealing.
I know people will mention f/2.8 vs f/4 for stopping motion, which is a valid point… But UWA lenses in my experience require less shutter speed anyway to freeze motion, as objects move “slower” across the image circle than say a 70-200. Not to say you still won’t get that benefit, but the more I think about it… This lens could be quite perfect. Even if they came out with a 14-24 f/2.8 later on, I am not sure if I would switch… Especially given you can’t use filters with it most likely and I may rather buy a fisheye of some sort instead of extreme UWA situations.