Earlier quoted context omitted.
I wouldn't call that an open office though, just a team room. A shared space is fine with <5 people, can become problematic at 5-10 people and the frequently 20+ large floor spaces are just unworkable without noise cancelling headphones.
I work in an open office with about 30 people in it and find it unbarable on times. I have some good noise cancelling headphones that help sometimes but I can't wear them for too long a period before they start getting uncomfortable.
The other problem is people trying to do Skype or other conferencing calls while wearing closed-can headphones. Their own speech level rises two notches when they can't hear their own voice. Try it for yourself and see.