I usually hate shit like flash mobs and Improv Everywhere. It epitomizes the most annoying thing about bad theater: It is more about the performer than the audience and it takes up more time, than say a bad painting or something. Also, I think that audiences should have the right to consent to viewing a performance. It shouldn't just happen to you; you should be asked first.
Anyway, I obviously think about this stuff too much. But all that said. I love this video by Beirut, even if it just happening on the street.