I had some friends visiting the other evening who were interested in checking out Heavy Rain. I found it a slightly bizarre situation, with one person playing and another three watching. It's really not the usual way to play it. It was very easy to laugh at mistakes and diffuse the seriousness considerably. At the same time, I think they were surprised at how much tension the game managed to include, even while we were joking around.
I was a bit surprised, I admit. As someone who is rather easily drawn into game stories and emotions I have trouble gauging when something hits the level where it might actually work on other people, too. Judging by the comments I've seen online, Heavy Rain comes close enough to affect a fair number of people, but that still leaves a significant number of people who will find it clumsy and lacking in true maturity and immersion.