To be fair, plenty of people like it. I hate it. It's a personal preference. In terms of anything that's really quantifiable, I'll say that I believe "open plan" leads to a constant stream of distractions and interruptions which kills your ability to get into - and stay in - that "flow" state of maximum productivity.

It also sucks if you need to make the occasional personal phone call, or if you feel the need to pick your nose, etc., etc.