I've been using Plone for my company's Web site (www.lerner.co.il) for the last two months, and I am amazingly impressed. It's trivial to install, trivial to customize, and the documentation (as Eric mentions) is quite excellent.
Plone is built on top of the CMF, which is a set of products built in Zope. If you think of Zope and OpenACS as analogous, then CMF and .LRN are somewhat analogous as well, providing additional infrastructure for the creation of new applications. Continuing with this analogy, Plone is thus similiar to a package that depends on .LRN.
I don't think that I would recommend Plone for serious dynamic sites or for those that need database access. But it's an excellent tool for what it's designed to do. And as Zope's CMF catches on (which may happen one day, when the documentation is understandable to normal humans), I expect that there will be more types of dynamic content within Plone.
Oh, and one more thing -- Plone is very easy to use, even by someone who isn't a Web/database hacker. Very little documentation is necessary for new users.
More to come in my LJ columns during the next few months...
