I just FaceTimed my parents and did the peace sign, worked great. The cool part is the real time cutout that puts balloons *behind* you as well as in front.
Can't really do the heart sign when holding the phone with one hand. I think they should have a button that lets you manually trigger these reactions on the iPhone -- in fact that's what I was searching for before remembering it's triggered by hand motions. Another issue is I don't know which reactions exist because there's no guide anywhere, effectively making this feature an easter egg.