RxJS Observable with initial value
This is a really good question -- starred it! I am not very experienced with RxJs, but I believe your code around BehaviorSubject and Observable is as concise as possible.

BehaviourSubject itself is "subscribable" (just like an Observable) but it's a really bad practice to expose it directly to the consumer, i.e. your separation of users$ and usersSource is a right thing to do. Exposing Observable is okay because it's designed to be read-only.

usersSource.getValue() is inevitable in your code, unless you decide to keep the usersSource state as a local field, but IMO it does not do any good in this scenario. Having state to be stored as a part of the BehaviorSubject is handy.



There are minor style things that could be improved in the code, but that's unrelated to RxJs, so I'll not comment on that.



Update 1

Just a few months ago I was not aware of .startsWith() which seems to be an answer to a part of your question:

const neverButStartsWithZero = Rx.Observable
                                .never()
                                .startWith(0);
// Emits 0
const subscription1 = neverButStartsWithZero
                        .subscribe(value =&gt; console.log('subscription1:', value));

const threeEventsWithExtraZero = Rx.Observable
                                .of(1, 2, 3)
                                .startWith(0);
// Emits 0, 1, 2, 3
const subscription2 = threeEventsWithExtraZero
                        .subscribe(value =&gt; console.log('subscription2:', value));