I am such a worker, and my immediate boss sits literally right behind me. It's mildly uncomfortable, but not really much more uncomfortable than a traditional set of cubicles. It helps that my boss doesn't care if I'm e.g. reading this site instead of working at any given time, as long as I get my work done overall.

I estimate I would have about a 50% increase in work done if I had an office with a door, no increase if my boss was not in the same building and I had an open plan office, and no increase if I had traditional cubes (open plan offices really do make it easier to talk to people if you need to).