I have an excellently performing EF 16-35 F/4 L IS. However, I pre-ordered the RF 14-35 F/4 L IS as soon as it was announced. The only way I won't keep it is if it underperforms at similar settings with my EF lens. I am ready to do away with the adapter, and I want to have a full RF kit (this would give me 14mm to 500mm inclusive, with this lens and the RF 24-105 L and RF 100-500 L I already own). After that the only lenses to add for me would be a dedicated nightscape lens (first RF mount 14mm at F/2 or faster from any reasonable manufacturer will get my money) and maybe the new RF 100mm L macro, since I never sprang for the EF version. Since I shoot primarily scenic, with a little nightscape and wildlife thrown in I think that will give me a very nice kit paired up with the R5 for a long time, and should outperform my 5DsR setup I used for years (EF 16-35 L, EF 24-105 L, EF 70-300 L) in about every way. I am very glad for the extra 2mm and the IS - would not have considered the lens without the IS and appreciate the extra 2mm on the wide end. All that is left is to see how it performs, but I expect it to at least match the existing excellent 16-35.
ML