I've had my S4 for four months now and the battery life is great. I'm a heavy user and get 14 to 16 hrs of battery life. Using auto brightness with around 4 hrs On screen time. WiFi for around 14 hrs a day. Bluetooth and streaming Pandora for 2hrs. I have 3 email acct. That update every 4 hrs. 8 home screen widgets that update throughout the day. On the web a few hrs and doing a lot of reading for about 2 to 3 hrs.
I have a spare battery that I carry with me, but have never had to use it. (Better to have it then not to.) With moderate use, about 1 to 2 hrs on screen time, I can get 18 hrs easily.
When I play intense video games, Ashalt 7 or the Sims, battery goes down much faster.