I use plastic lensed glasses/goggles because I struggle using mesh in the dark corridors of the Malls. Tried it, hated it, end of.
This weekend I used a pair of the new Bolle Cobra full seal safety glasses with their new Platinum anto fog coating and I must say it worked exceptionally well. As previously mentioned I sweat like Jimmy Saville in a childrens ward so fogging IS an issue. Normal people would fine them utterly fog free.
Having read Ian's very techy but informative post I thought to myself "I'll go check the numbers on the Bolle stuff, see if I can prove a point". So let's do the math(s):
Let's say you have a gun set right on the site limit for the Mall. 340FPS on a 0.2g BB. This gives you an energy of 1.07 joules
Bolle safety glasses have a maximum impact rating of "F" according to the EN standard of marking which is tested with a 0.86g steel ball at 45m/s. 45m/s is 147.64FPS which then gives you an energy of 0.86 joules.
So as you can see the glasses are insufficiently rated for even a fairly conservative site limit. 0.2 of a joule may not seem like much but if you were unlucky enough to be shot point blank in the face (which DOES happen in CQB) then your eye protection may fail.
Bolle GOGGLES on the other hand are rated "B" which is tested with a 0.86g steel ball at 120m/s (393.7 FPS) with an energy of 6.16 joules.
The moral of this particular tale then is wear goggles, not glasses (from Bolle at least) and make sure you WEAR THEM PROPERLY. You can have the best eye pro in the world but if your face protection doesn't let them fit properly you may as well not bother.