I strongly suggest you stay on the stock firmware. Nowhere near as much refining & testing goes into updates compared to the original optimized package. When people update they are gambling the features they use/depend on (gps, wifi, BT, MMS - they can break with an update along with many other things) to gain trivial ones they won't. Never worth it.
The way lollipop handles sounds/notifications alone is a disaster and that's before any bugs lollipop may give you.

 

I've just updated and my phone is running like a treat compared to before. My battery life has improved, the phone looks nicer and it is a better overall user experience. It feels like a new phone. Only thing I have found on my phone is that there is a bit of lag sometimes when I long press the home button to bring up the open apps.

I'm glad I did it. It might be different for other users but I'm glad I did it.

 

Settings>General>About Device>Software Update.
Unticking auto check for updates should do it.

 

I updated to Lollipop several weeks ago. Till now, I think it works fine.