Many companies keep their org charts a secret to dissuade headhunters from poaching important employees and to hide details that could give competitors insight into product development. This has led to corporate intelligence companies that research and sell org charts for lots of money. So Cogmap is a big deal...in concept.
Unfortunately, you get what you pay for. Cogmap is wide open. Anybody who registers can go into an org chart and rearrange who works for whom. While the Ajax service is nicely built and very easy to use, it just encourages people to drag reporting structures around, or to create org charts that are speculative, not based on research. There are wiki-like versioning features so the community can police itself, but it's much harder to spot errors on an org chart than in a text article on Wikipedia. You can't really be sure that any chart on the site is accurate or timely.
That said, as Rev2.Org points out, Cogmap can give you a useful, top-level view of a company's overall management structure. Also, if you want to create a quick org chart of your own company, it's a nice utility.
What I'd really like to see, however, is Cogmap mashed together with TheyRule, the (now dated) site that shows you the structures of public companies' boards of directors. That would really help you understand the power structure of a business.