I have recently taken ownership of a reasonably mature application, but the fact that it has had a large number of developers and lead developers over time as well as a lot of historical decisions with no form of testing framework all adds up to an application of unknown quality.

I used Eclipse (WTP 1.5.4) to clean most of the warnings and got down to less than 200 from over a thousand. Mostly unused variables and imports, no stress there.

Having used FindBugs before I thought it would be a good idea to give it a look at our source, and I was happy with what it found. It found bugs\

Some of them were the sorts particularly hard to solve, such as a semicolon at the end of an if statement. Others, such as java.sql.Statements reading from zero rather than one would have been found eventually, but it was nice to find them first.

No, it didn't change the world, but it definitely provided an additional, useful, view of the code.