Highland is a great option, even easier to use than Caolan's previous library Async.js.
Re: Highland vs. RxJS, RxJS operates at a lower level interface [0]. As a plus for RxJS, Reactive Extensions are available for multiple languages, not just JS [1].
RxJS author here. Right, an advantage is if you know the Java, .NET, Python, C++ version, etc, then you know the JavaScript version. That and we try to adhere to an Array#extras style following all of ES2015/ES7 Array methods.
Another advantage is that many frameworks such as Angular 2.0 are shipping with support for Observables, so that's a huge win for users of RxJS that it automatically just works.
Another advantage is that many frameworks such as Angular 2.0 are shipping with support for Observables, so that's a huge win for users of RxJS that it automatically just works.