Has anybody used the Client/Server Product ?

Yes its excellent and I wish time to help develop it :(

If I create a dtml document on the server, can I use it through the client ?

No, it facilitates the moving of objects from one zope to another. Think of it as a more advanced import / export feature.

is the server just a repository that can´t make changes ?

No the server can make changes, but then it could be out of sync... -- Andy McKay, Developer. ActiveState.