|I used ClearCase for years, both as a user and as an administrator. It has got to be the most convoluted, God-awful RCS on the planet.
Its biggest drawback is that everything occurs over the network. You create a virtual drive that points to the versioned object (VOB). Files that are checked out (locking system = bad) are local to your machine. Everything else is coming from the server. Builds are slow as hell.
Multi-site support requires someone to be a "merge master" and put the right bits in the right places every morning when the remote packet is merged. Been there, done that, not going near it again.
Subversion is a much better product. Smaller, easier to administer, and need only rely on HTTPS instead of custom protocols. Since every engineer is responsible for merging his own changes, no one has to be a merge master at 0700 before people hack on the code for the day.