The question facing everyone is whether to use an application-building tool or "framework" to build your own CMS, or buy one of the many proprietary finished products in use by major web sites, or simply rent a CMS from an Application Service Provider and avoid all the headache of running a web server in your own organization.
Build - Tens of thousands of CMS developers are using open-source application frameworks like Zope or PHP, as well as proprietary tools like Cold Fusion and Active Server Pages, to roll their own content management systems. Since everyone's content can be seen to have unique needs, it seems to make sense to tailor the CMS to match an organization's content.
But this is a little like everyone programming their own desktop publishing tool. There is a reason why PageMaker and Quark Xpress dominate their industry. Much of what we publish is similar in presentation or style to other publications, and standard tools facilitate productivity (availability of trained workers, etc.)
If your content is truly different, then build your own CMS. But keep in mind that you may be more attracted to the technology (the medium) than to your content (the message). You would be better served by getting an experienced open-source CMS consultant to build it for you.
Far too many of those building a CMS think that they will sell it to the rest of the world and make their fortunes. It's a buyer's marketplace, but there's a very high chaff-to-wheat ratio.
Buy - Very few industries have been racked by the dot-com bubble as much as CMS suppliers. They were enticed to provide sophisticated websites to all the dot-coms. Budgets were fantastic. Schedules were unrealistic. Claims were hyperbolic, and eventually the backers went ballistic.
But in the short term huge revenues flowed into market leaders like Vignette, Documentum, Interwoven, and Broadvision. If they have as many failures as success stories, the cause was very likely these impossible market pressures, and not the suppliers fault.
The vast income has been used to build (with research and development) and buy (with mergers and acquisitions) major technology improvements, so surviving companies are likely to provide solid products in the coming years of CMS realism.
You might also "buy" an open-source CMS by going to the many independent CMS consultants who will build it to your specifications.
Rent - A few CMS designs have become well-supported and popular web applications (notably AtomZ and CrownPeak). All the production software is at a huge central ASP server with high-availability Internet connections. This central server will deliver your finished pages to a production server, at your own or another hosted web server. You benefit from constant upgrades of the toolset that happen transparently, and need only train your workers to create and maintain their pages remotely.