I've had a chance to get involved recently in a JAX-RS project in Apache CXF. This project is far from being complete yet but that's another story... I have to admit I've had a very positive experience while working on it.
CXF JAX-RS has entered the game quite late, with the leading implementations like Jersey, and very likely equally strong RestEasy and Restlets JAX-RS being there for a while already. Thanks to the work of my former colleague Jervis Liu, the feedback from the CXF users and the tremendous feedback from JAX-RS experts and the very open process behind the way JAX-RS and Jersey RI team operates has helped CXF JAX-RS to catch up a little bit with 0.8 api mostly supported now.
One can wonder what is next for CXF JAX-RS ? I hope that with the help of the large CXF community, its JAX-RS implementation will eventually become as solid as its top-notch (CXF) JAX-WS 'counterpart'. Will it be able to compete with the likes of Jersey, with it's client api gem (which will hopefully get standardized one day :-) ) ? It's hard to tell - not in the nearest future for sure.
Still I reckon CXF JAX-RS will eventually find its niche. While CXF JAX-RS users can certainly write RESTful service only with no JAX-WS (SOAP) involved, possibly its best 'argument' is that it sits on top of and is closely integrated with the same runtime which hosts quite possibly the best JAX-WS implementation out there. CXF users will be able to write the services combining JAX-WS and JAX-RS technologies, with one or more closely related Java classes involved, some will experiment and decide to move to JAX-RS altogether, some will use the combination of two technologies and some might get back to JAX-WS, you never know :-)
Generally, I'm somewhat pessimistic about Java code relying on various annotations which is what JAX-RS is all about. Having said that I have to admit that JAX-RS is much more involved in that it's not only about the annotations. Possibly the main achievement of JAX-RS authors is that JAX-RS 'breaks' the barrier for Java users at large and let them experiment with RESTful services while still being in the comfort of the their JAVA code. It's a great effort by those behind the specification and the RI.