The phone is rubbish with stock Samsung OS and software! Slow often glitchy, long waits for simple stuff like switching between apps or even going back to the home screen.
BUT - with a custom ROM this phone is potentially AWESOME. Lag is gone, battery life much better. I make my girlfriend jealous and she has an iPhone 5S, lol.
Slightly bigger screen than iPhone, much lighter, more sturdy, expandable memory, FM tuner, and with a custom ROM loads more features and better interface.
Some ROMs are not available for this phone but a lot are: Many available from maclaw.pl, I've tried CM11 but found Carbon (latest unofficial KitKat) has a few more options and some nice optimisations to maximise battery life without sacrificing performance at all.