The S4 looks cool, but the biggest turn-off that I have with it, is the huge screen-size that makes one-hand typing impossible. I have the impression that Samsung was trying to cross the features of a smartphone with the large screen of a tablet - but, it just looks and feels really awkward in hand. Personally, if I need a bigger screen to watch something like a movie, I'll use a tablet or a laptop - not a smartphone. My contract with T-Mobile expires next week, but I'll wait for news on the iPhone 6 in September before upgrading.
I've gotten used to using iOS on my iPad, and I can say that it's a significantly smoother interface than Android. The lack of Google chat is something that I do find annoying, though.