Open office does not scale beyond 6-10 people. There was an article though about a firm that did have open office but had special function areas - like quiet work area or meeting room are. If you are in quiet room area, you don't talk there at all. Huge open office I think is bad as it invites distraction.

Many companies grow from open offices as startups but then they have to change they way they do the open office if they want to preserve the "open office". my 2c.