Samsung Galaxy S4 [GT- I9500] - Android 4.2.2
I would definitely recommend you upgrade to Android L / 5.0.2. 

There a numerous changes relating to performance, new features, and battery life. Android L gives a completely different (better, in my opinion) feel to the phone and fixes many of the bugs from previous versions.

To answer your question about including new bugs; it's inevitable. But if they are addressed, it's highly likely they will be fixed in the future.