I really enjoyed it! It wouldn't be for everyone. You better know the person well before you pull something like this.
If anyone is interested, here is a link to a flash mob my sister (the CM) was in at Disney's Wide World of Sports about a week ago. So many kids in the area joined in that it becomes hard to see the real choreography, but the kids seemed to have a blast.
My sister is toward the back to the left side in tan Capri's.
http://www.youtube.com/watch?v=rMLNNvy-uo0