I've recently used Jersey and found it very easy to use, especially in combination with JAXB. You put some JAXB notations on your beans, some JAX-RS notations on your resource class and then you're almost done already to expose methods in the resource class as RESTful web services. Jersey and JAXB not only support XML, but also JSON - the only thing you need to do is change the MIME type setting and it will automatically generate and parse JSON instead of XML for you.
Jersey has a good user guide and another nice thing is that it has a client API as well as a server API. I've used both, using the Grizzly embedded HTTP server for the server part, which is also very easy to do with Jersey.
Jersey is certainly stable and complete.