While I value the implementations of JWebsockets and Autobahn a lot, I prefer Atmosphere.
Ramp-Up::
Ramp-Up time is low. Francois Arcand puts a lot of effort into testing and examples, helping everyone to achived quick wins. (And I have no clue why he's able to respond so quickly to any architectural questions I rise. Impressive.)
Maintenance Perspective::
To me, maintainability is fundamental, if a software goes beyond major release version 1.0.0. The project is maintained at a top Maven-POM level with a properly constructed hierarchy. It prevents library incompatability. Libraries are reference at the correct level. That is well done.
Technical/Functional Perspective::
It offers a Java SE client library (wasync) that can either do native websocket communication (onMessage) or build on Jersey REST annotations (@Path). Subsequently this makes it as simple as it gets with the benefit of maintaining an open connection for subscriptions and the popular remote procedure calls (RPC) paradigm. It is a common attempt to merge those two paradigms. See also http://wamp.ws/, which heads for the same approach. Also, the library offers to establish QoS properties like robustness (e.g. in case a client disconnects) and reliability (caching undelivered messages). That is great for professional software to harness.
