A very good overview by James Robertson on the pros and cons of using an open source content management system.
Community-based CMS: these systems are best suited for organisations that have strong internal development resources, as customisation will need to be conducted in-house (in the absence of commercial support). This makes them unsuitable for any project requiring ‘out of the box’ deployment.
Commercially-supported CMS: these systems should be evaluated like any other commercial product. While the licensing cost is zero, the system must match business needs.