Good news for CXF JAX-RS users: Andriy Redko has opened a CXF Master JAX-RS 2.1 branch. Server Side Events is the first feature of 2.1 API which is supported on this branch. Having this 2.1 API Snapshot is handy.
The development of JAX-RS 2.1 has been frustratingly slow but there's some progress nonetheless with Jersey (RI) expected to be ready as soon as realistically possible, given that all the major features proposed for JAX-RS 2.1 have already been implemented in Jersey.
JAX-RS is easily the best API for building REST clients and servers. Despite the process difficulties it will continue evolving. Use it and believe more is to come in the JAX-RS space.