What do you think about TDD (Test Driven Devolepment)?

When I started my career as a developer, I was flooded with many articles about best practices, and one, in particular, caught my attention: the TDD.

In my case I used it a lot when projects were small and new, but now working on larger legacy code I can't use this concept.

My reflection here is around the real gain, and your experiences using that in different types of projects.