Often I am told that it is difficult to use Agile/Scrum approaches for brown field projects or for big projects or for distributed projects or for in other situations.
Interestingly these persons also state that the same problem exist for RUP/OpenUP, Waterfall model DIN XT, Swiss variant called Hermes.
Currently agile approaches are the standard for new projects and are thought in technical universities.
Scrum is the main agile method and state of the industry approach for the implementation of software projects.
To open the discussion and bring the discussion back to objective arguments I often recommend the following books
