I've had limited experience using unit testing, but I have read a considerable amount on the subject - especially as it pertains to Java. Most of the time, I want to add unit testing, but it's always a hard sell to management. It's an even tougher sell to management when you come in to a project that has been selling. After all, "The product works fine. Sure, it may have some bugs, but we'll fix those in due time.". I come from a more proactive school of thought where I'd rather we find ways to prevent bugs in the first place, and have the code be maintainable enough to fix it when a bug does creep in.

That's where unit testing comes in. If you can find a way to automatically identify bugs before they get into your codebase, you're much better off. Additionally, writing unit tests typically results in writing more maintainable code. After all, I would hope that you wouldn't want to write one test case to exercise a 3,000 line function. :-(

All of the above is just an introduction to John Kaster's BDNTV episode on unit testing in Delphi 2005. Check it out. It's short, but still very informative.