SIP is a very flexible, lightweight protocol for making multimedia calls. We've mostly talked about telephone-style calls because they're most familiar to most people, but SIP really comes into its own with user mobility and heterogeneous multimedia terminals.
It is not yet clear whether SIP will see widespread deployment, or whether the ITU's H.323 protocol which performs a similar task but is more heavyweight and less flexible ends up becoming the de-facto standard. Hybrids of the two are also possible. Time and the market will decide.