+1 for Grokking RxJava, really great set of intros, especially from an Android perspective. Gonna pile my two cents on this as well. I think the easiest place to add
RxJava in an existing Android app would be your networking code. Square's Retrofit works really really well with Observables and you can do some very interesting things that imo are more immediately graspable than say writing reactive UI code.
https://joluet.github.io/blog/2014/07/07/rxjava-retrofit/
This is a short post that I think nicely shows how Retofit+RxJava fit together