leocharre is correct in that tests reduce development time in the long run, but it also reduces development time in the short term. By writing a test, you figure out how the code you're about to write is intended to be used. That will help focus your thoughts.
Basically, something that takes 5 days without tests will generally take 3 days with tests, including the time to write tests.
My criteria for good software:
- Does it work?
- Can someone else come in, make a change, and be reasonably certain no bugs were introduced?