title: Observable.first().subscribe() in place of Promise.then()
body: <p>I have a variety of methods which I recently converted from returning promises to returning observables. Currently, these methods usually only return a single value. So, where I used to call <code>myMethod.then()</code>, I switched to <code>myMethod.first().subscribe()</code>. I did this instead of just <code>subscribe()</code>, because I'm hoping to have the observable and subscription automatically complete and clean up. Is my understanding of this correct, and is this a reasonable approach?</p>