there is no gaurantee that L lens wont break nor there is no gaurantee that a 500$ Sigma will definitely break on a fall. In fact the EX signifies the "L" level build quality and except for the flakes on the outside the sigma lenses are really well built. I have no second question about that.
The sigma EX lenses are not Plastic lenses and john was talking particularly about ex lenses
I feel the EX grade sigma lenses build quality can be comparared to L build quality.
the sigma 150 mm macro feels like a stubborn piece of metal. Its strong enough to dent a person trying to steal my copy