For use walking / light hiking, they do fog up
I use these goggles for walking and light hiking in the northeast during the winter. They are not used for skiing. In this application they do fog up during certain weather conditions. If there is a strong wind they won't fog. Otherwise, they do. I honestly can't tell the exact weather conditions in which they fog or don't fog. It's sufficiently annoying that we're trying other goggles (Smith I/OS and Smith I/O) which a BC gear expert says do better at not fogging and are even more reliable and better performing than the Smith Phenom goggles with a fan.
I will say that other than (!) the fogging, the vision is great and the adjustment to light conditions works very well. So for downhill skiing applications where you have a strong breeze to create ventilation they may be great. But not for walking or light hiking.