I have consistently had some problems on big snow days in Tahoe with my goggles fogging up. It seems that I am fine until I start to sweat. Once any perspiration builds up in the lense it is party over and this has curtailed my visiblity (and fun) on big pow days. The upside is that I have learned to ski by brail.
When I was with Heyoka in Utah he showed me his Smith Turbo Goggles whcih are made to be worn over glasses. They have a fan in them that when running elimanates all fogging. I wear contacts and I was a little concerned that the fan would dry them out.
I went to ski in 3 feet of fresh Tahoe snow (and it was light and fluffy for a change) on Friday. I can now say unequivicably that this is the best $130 that I have ever spent on equipment. Everyone I was with had a fogging problem at some point in the day. I had none. I kept the fan on low the whole day and never had an issue. I also had a high speed to go to if i ran into trouble. Even better, it comes in a rose shade which I find to be the best in flat light.
This product is just plain a home run. If you ever fog, buy these and say goodbye to the issue. For those that see big dumps of fresh snow often (Tahoe and Utah) for many these are the difference between enjoying a full day of fresh pow vs. only enjoying the first half of the day. JB.