After a few month, uning JMeter for my functionnal/integration tests and having included them in the continius build process, I can see major changes in every day work.
First (I knew this one would come up that's why I did it ;-) ) there are fewer bugs (almost none), more self confidence in code and application stability.
Secondly (I hadn't thought of this one first) It helps refactoring since you gain testing time. I can now use this time for refactoring.
Everybody at work says I'm "test addict", I think back then when I hated testing (but did it)...
Automatisation of tests is far more interesting!