Pro's and Con's of unit testing after the fact

Question

I have a largish complex app around 27k lines. Its essentially a rule drive multithreaded processing engine, without giving too much away Its been partially tested as it\'s been

Answer 1

Unit testing is still definitely useful. Check out http://en.wikipedia.org/wiki/Unit_testing for a full list and explanation of the benefits.

The main benefits you will gain are documentation, making change easier, and it simplifies future integration.

There are really no costs to adding unit testing except your time. Realize though that the time you spend adding unit testing will reduce the amount of time you will need to spend in other areas of development by at least the same amount and most likely more.