While I agree that this does indeed seem gimmicky and will most likely be just that for the majority of HD DVD users, for me it'll probably be pretty neat.
Every so often I do this manually with my brother, and then talk on the phone or over Xbox Live throughout the movie. It's sort of fun when you live thousands of miles from each other and only get to meet once every couple of years. The problem we usually run into is that one person is ahead by a little bit, or if one person pauses to point something out the other is usually not on the ball, etc. This will take care of all that.
If he could just get around to getting an HD DVD player we'd be all set.
KM