Question

When will you use dependency injection? Is there any overwhelming benefit of using dependency injection?

Answer 1

Fowler has good comparision between the two in his Inversion of Control Containers and the Dependency Injection pattern Heading. In his concluding thoughts, he says

Dependency Injection is a useful alternative to Service Locator. When
building application classes the two are roughly equivalent, but I
think Service Locator has a slight edge due to its more
straightforward behavior. However if you are building classes to be
used in multiple applications then Dependency Injection is a better
choice.

You can find more view points and comparisons in here

For DI:

• Easier to determine what dependencies a component has – look at
  constructor.
• Component does not have dependency on Service Locator so
  there is not a problem if the component is used with a different
  framework.
• DI may make testing easier but a good Service Locator
  mechanism will make stubbing equally feasible

Against DI:

• Harder to debug and understand.
• Component cannot request extra services from injector once it had been configured.