When using iChat Server, text chats are routed through the server (that is, clients send messages to the server, which passes them onto their destination). Video and audio chats, however, rely only on the server for initial connection. After that their connection is point-to-point between the clients, and data is not streamed through the server. The same is true of file transfers. This reduces the network load on the server.
Also, Apple has designed iChat Server as an internal solution only, meaning that it will not attempt to exchange messages with other Jabber servers (either on the Internet or within your network). Mac Geerkery has published some information on how to modify iChat Servers configuration files to interact with other Jabber servers.
Finally, iChat Server does not log chats (some third-party Jabber tools do exist that can add this, as well as other functionality). It does, however, log user login and logout. This can give you a good idea of which users are using your server—and for how long.