Dependency Injection: What is it? Do I need it? Can I become addicted?

Symfony2's components work as a set of services that follow the principles of an SOA (Service Oriented Architecture). The main thing gluing all of this together is a concept called Dependency Injection. Let's get together and discuss the topic, making sure that we highlight the pros and cons of using it and how we can use this powerful development pattern to our advantage in our own projects.