I actually really like TWiki. It's been very stable for us, and is super easy to use.
Here are some benefits:
- Uses rcs instead of a database in the backend, which simplifies administration and backups
- Access control and permissions are embedded directly into wiki pages ("topics" in TWiki-speak) using the same markup
- Namespaces via "Webs" help to organize content
- Tons of features are shipped by default (e.g. email notifications, WYSIWYG editor, versioned attachments, very flexible access management) and tons more available as plugins (e.g. Topic tagging, collapsable tree-views showing the semantic relationship between topics)
- Pretty by default, though this is very subjective