Never had one. I rode and raced dirt bikes and motorcycles since 1983. Crashed a lot and broke several bones, tore things, broken back too. The airbag *may* have saved me from a broken back on the motocross track but... unless it can prevent over extension of the spine it would not have.
The rise and fall of the Leatt neck brace says most riders prefer the unrestricted feel of being able to move around naturally on the bike rather than the constricted feel of these safety systems. I get it, safety first but if it slows you down it's not worth it in professional competition.
There are riders in supercross wearing the airbag and I have seen one deploy on TV, whether or not it helped they didn't specify.