My longest experience with an open plan office fit all of your bullet points: we all wore headphones, we used IM before starting conversation (or in place of vocal conversation as often as not), we had a private meeting room we could grab just across the hall, and everyone got alone with each other really well. Still none of us liked the arrangement.
We all felt that we were less productive, more distracted, and that there was always someone looking over our shoulder. Management actually went out of their way to stay out of our room, but the feeling persisted. Management knew we didn't like it, but I don't know if they knew how much it hurt productivity. We didn't talk about it. Nothing was going to change. They decided long ago that we didn't have room for private offices. Why complain and get on bad terms with the boss when it won't change anything?