Already been answered afew times before - they're extremely dangerous. They provide no protection against BB's as the plastics used will shatter upon impact, and the gap between the lens and your eyes is huge.
That is meaningless. What if another player is using 0.5g ammo or higher?
If eye pro can't take a .4g bb at 350fps (~2.5J), it's not safe (this is the highest you're likely to encounter in an airsoft game if everyone's following the rules but with joule creep i'd play it safe an add a joule).
You should assume there is some one using .4g ammo with a gun which might break at any moment at shoot you in the face on full auto.