Open offices instantly "out" anyone who comes late, leaves early, takes a 2.5 hour lunch breaks, or surfs the web instead of working. And that's really the whole point. It's why they are popular in Japan, together with the fact that thanks to the open structure, transgressor can apologize in front of everyone at once.
Looks at HN karma. Yeah, if only they did that.
I guess a lot depends on company culture, but I'm yet to work in an open-office space where I could actually focus on anything harder than the most mechanical coding tasks.