This milestone release provides a couple of significant new features to web3j.
Firstly the JSON-RPC client API has been enhanced to integrate with RxJava's Observables. In addition to web3j.< methodName >.[ send() | sendAsync ], you also have observable() which will wrap the request in an observable, allowing straight forwards composition of JSON-RPC calls.
Secondly is the addition of web3j's filter API. web3j provides a fully asynchronous API for working with Ethereum client filters, so you no longer need to poll the client for filter updates. For instance, receiving a callback with all new blocks or transactions being created in Ethereum is very trivial. For more details refer to the new Filters and Events section in the documentation.