At first I didn't like that I need to write tests for my code, but now after using it for more than two years I can't program without it.

Helps to come up with better APIs.

It gives confidence that our software is working well at all times. Even after making major changes and/or changing software we are dependent on.

Productivity increases - you might loose some when you make the initial tests, but you'll get it back later. The code covered by tests is 'insured' against future changes.

It works well for libraries, not so well for GUI applications.

Some things cannot be tested like server errors, unless you use mock-objects.