Importance of unit tests varies with the projected lifetime of the software you're writing. If you're writing libraries or system services, then unit testing is

Importance of unit tests varies with the projected lifetime of the software you're writing. If you're writing libraries or system services, then unit testing is smart, as those are bound to live on for many years. Simple end-user applications might be a different case. On the systems side (in this case, a PHP framework), I've found that once test coverage passes 70% you can make optimizations and refactor the code without having to actually run it - the tests will tell whether the changes you did still maintain API stability.