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

Depending on how many bugs "manual testing" turns up, you could simply do test-driven bug fixing which in my experience is far more effective than simply driving up code coverage by writing "post-mortem" unit tests.

(Which is not to say writing unit tests afterwards is a bad idea, it's just that TDD is almost always a better idea.)