I recently bought an S4 (I work for a provider and could get the extra handset repayment for only $20AUD a month on top of my regular bill with my i5 so I jumped) to see how the device was. I didn't want to make the jump to it as my only handset, be locked in for 24 months and then regret it.
So far I am on the fence. There are some things it does fantastically well (camera, screen, customisation) and others I'm not thrilled with.
The things that I am missing the most is the app suite (granted that Andriod is
nearly there, but not quite) and the integration my iPhone has with my car.
I could just buy an iPod to stick in my car but I shouldn't need too when my phone does it for me as is.
However the screen is amazing, I love looking at it and using it. Colours are vivid and the size is just great. If I stick with the S4 I'd actually sell my iPad Mini and upgrade to a bigger iPad just becuase I can't see myself using the Mini as I do now because the S4 would replace it.
I'm also hanging out for the iOS 7 reveal as iOS as it is has become stale and boring to me. I don't want radical changes as it works very well but I would like a different feel or some more minor customisation options (better notification centre, quick toggle option for wifi, mobile data, DND, etc).
Both are very good phones, just need to make a deicison as to which will be my daily driver now