I went and had a quick look today. As I mentioned, it is built on top of Zope, which is itself a server/development environment/lots of other things that is written in Python.

I imagine Plone has lots of the same benefits as Zope — object-oriented, built-in management of permissions/roles for users and groups, etc., as well as the same drawbacks — must be run on top of Zope, steep learning curve, low availability of skilled Python/Zope developers.