My first ever useful website (http://www.newmetalarmy.com) had only a few unit tests and it's a real fucking pain to change one year after launch without breaking anything.

Now on my next site (http://www.beepmystuff.com) I've got many unit tests and I'm much more confident when I make large changes.

The two site share a lot of code so the first is benefiting from the second getting a bit on TLC.