Is Puppet useful when you have installed all unique servers in your organization?
Yes, the puppet is useful in that case too. All servers are unique and similar in some ways. You just have to analyze the details completely like operating system, configuration details, security setting etc. Note the similar things in one file and unique details into another file. Write Puppet manifests accordingly to automate the administration tasks together in a single pass. Each of the fact details is defined as the variables in the configuration file and they can be used further to handle the uniqueness.