The promises of open-plan offices are that it makes everyone more available to help everyone else, and makes it easier to work with your neighbours. The reality of open-plan offices is that being available to help others means you are more likely to be interrupted and unable to get your work done, and even if you aren't the subject of an interruption, you will be aurally distracted any time coworkers strike up any conversation, work-related or otherwise. Open-plan offices have been shown experimentally to be terrible for productivity, and employees must usually deal with the overwhelming noise by drowning it out with headphones, further harming productivity.
If you want your workers to be productive and to cooperate with each other, almost any office layout will be far better than open-plan. Just keep it in mind if you've got an office on the drawing board. Open, flowing space is good for parties, not for getting work done, for identical reasons.
Mokalus of Borg
PS - I'm not a fan of open plan.
PPS - Unless you're throwing a party. Then I'm in.