“RE: build or buy?”

Each application has individual needs, but also has needs it shares with many other applications, as Greg pointed out. It makes sense to share the development costs for the common stuff with as many people as possible, since that means less money for a superior product. The unique stuff will need to be custom-written. Going with a platform that is easily extensible, and which is ideally open-source, gives the most flexibility. Paying for the time of people who are already intimate with the platform will usually be cheaper than paying for your own people to get up to speed on a new platform, or to go through the trial-and-error of writing a system themselves and then building upon it.

In much the same way that it's far cheaper to buy Word and add custom functionality with VBA than it is to develop a word processor yourself, if a CMS system already exists that does most of what you want and can be customized to do the rest (and they don't gouge you like Vignette does), any project of a moderate to large scale should come out cheaper using the product than building it oneself.

Sean