For me, open office + headphones = private office. At a vastly reduced cost. I know that doesn't work for everyone though.
1. I have glasses and a hearing aid (cochlear implant) and have yet to find headphones that don't make my left ear/head throb with pain after more than six hours.
2. Visual distractions: people walking through, people seemingly approaching me (but actually approaching someone nearby), people waving, etc. are all just as bad as auditory distractions, if not worse for me personally since I tend to rely heavily on vision to replace what I can't hear.
As far as noise goes, even with audio engineers on staff, the level of disruptive noise was horrible. People having calls at their desks, testing mobile notifications, etc. Thankfully, I worked at home 50+% of the time as getting productive work done in the office was incredibly difficult. Large open space + open office = bad idea.
Our office in Stockholm, while an open office, had many smaller sections offset by walls, break rooms, etc. It was a much better experience overall, but still not optimal.
Also, what happens during flu season? How many people are gone and how often vs private office setting? I'd love to see a study on this.
I am working in a company that is redesigning the building to be more open plan. It is clear that HR is driving the decision not architects or interior designers.