The only ones I've ever seen have sort of a hanging glasses lens in them and (at least I would think) pretty much defeat the purpose of them, fog. I can't go without my glasses on the hill, but after a few years dealing with fog I finally just went and got contacts.
Now, I hate contacts, they bug my eyes, I have a hell of a time even getting them in, etc. On the other hand I can wear any goggles I want, I can see, and it's about a hundred times better than any OTG system I've used in terms of fog.
Really I only wear them on the hill, and that 8 or so hours from the time I leave till last tracks is not long enough to really start bothering me and even if they do I don't notice them on runs really, just when I'm taking a break in the lodge or whatever. Still amazing that I can see everything crystal clear with no fog, so worth the minor irritation IMO.