I work for a company whose entire office is on an open plan. Yes, even the CEO sits in a cubicle, as do all the senior management and stuff. And this is not a startup, mind you, its a mid size company.

Although it was kinda scary at first, I've adapted to it, and can see the practical benefits. Not having special offices, there is a sense of egalitarianism amongst the workers and none of the usual rat-race for better offices. As pointed out in another comment, most employees tend to use headphones when they need privacy. Since all the cubicles are the same, maintenance is a breeze.

I guess it depends on your temperament. If you really just like to be left alone when working, it won't help. For me, having so many people around kinda prevents me from going drowsy, which used to happen a lot at my previous job.