I belive beryllium is dangerous during the manufacturing process but left alone is stable. It is brittle and can be poked with a finger which is why there is a metal grill over the beryllium mid/tweeter of my TAD's.
I heard the $4000 Focal Utopia Headphones yesterday
Very nice. Very expensive. Beryllium drivers. Reviews a bit hyped I think. Who would pay that premium otherwise? Not sure I could live just with any headphones still.