If you are just starting your first serious symfony 2 project or you are learning symfony 2 and can not figure out which of configuration formats is the best one - answer is simple: XML

• XML configuration format is not so convinient as Annotations
• XML configuration format is not so understandable and readable like YML

BUT IF YOU ARE DEVELOPING NEW REUSABLE BUNDLE

• It will prevent your project from total refactoring;
• It will save your life;
• It will help you to make your bundle universal and upgradable;
• May be it will help to make your bundle framework independent too;

...and it is enterprice level

USE IT

• For routing
• For ORMs and ODMs
• For services
• For translations

Note: it is ok when you use yml in your root app/ directory.