Probably just like watching it in the movie theater. Since there is no positional tracking and nothing to give you any sense that you have any control may just work out like it would on the big screen. I wonder if anyone got motion sickness from just watching the movie.
I've heard that some people have issues with the movie. I think the issue with seeing the movie with a Rift on is that you would be looking at things from something that looks like a first person perspective, your head would be rotating and looking around but you'd have no control over it. I think if you couple that with you actually moving your head around in completely different directions, I think a lot of people would hurl.