Very recent informative discussion about it at:
https://optoutpod.com/episodes/s3e02-simplexchat/
Creator describes it as '"about halfway done" according to [his] own standards', so it has a long way to go to totally reach the vision. That said, it's already close to Signal parity, although not quite.
Despite that, it's already useful, and it eliminates the metadata / multiple account / separation of concerns problem that Signal has