As everyone has said, yes these do fog up, and badly sometimes. You should just put up with it - eventually the condesation will be so great that they'll become clear again, so don't take them off between rounds...It's just another one of the prices you pay to play the game. I've just recently pulled out the foam from the vents, but I haven't been able to see if it works.
-Easy to remove the black foam
-Easy to adjust
-Small, so you can use your optical gear easily
-Very nearly full visibility
-Fog easily (with the vents in)
-Depending on what kind of face you have, they may make you look like a complete and total geek. I know that I do. But I stick with these because they're just so damn comfortable (I've used them as eye-covers when I sleep to block out light)
Rating: [5 of 5 Stars!]