Closure is indeed TM-based, but that's not an A/B test of ease-of-use. I don't think there exists evidence to suggest that transactions are easier to use.
I think you are confused about the meaning of composability. The synchronized/atomic TM proposal for C++ is roughly single-global-lock. This means that you can still deadlock if you use synchronized to protect IPC with another process, which then does an IPC call back to you, which you service on another thread that also uses synchronized. Whenever these transactional proposals fall back on locking, they immediately bring in the pitfalls of locking.
The C++ TM proposal is a joke. There's no good reason to use concurrency if you won't get a speed-up, and the current design of synchronized is sure to make it more difficult to get speed-ups.
I think you are confused about the meaning of composability. The synchronized/atomic TM proposal for C++ is roughly single-global-lock. This means that you can still deadlock if you use synchronized to protect IPC with another process, which then does an IPC call back to you, which you service on another thread that also uses synchronized. Whenever these transactional proposals fall back on locking, they immediately bring in the pitfalls of locking.
The C++ TM proposal is a joke. There's no good reason to use concurrency if you won't get a speed-up, and the current design of synchronized is sure to make it more difficult to get speed-ups.