I spent over 10 hours testing that phone almost non stop, connected it to external amps, tv, tested the screen, chipset, reception, played games, checked camera. At the end I still had 56% of battery left! Easily twice as much as the S3.
The first smartphone that gives me this amazing battery life, beats the Samsung Wave I had over 2 years ago, which had a 3,3inch screen.
I also never had as much fun with any new phone I think. The software gimmicks sure are gimmicks but they make you feel like you're doing some jedi mind shit and totally enhance the experience of playing with your new phone. I totally only answer calls by waving my hand in front of my phone (although just placing it next to your ear automatically answers calls too), I scroll without touching the screen and skip music tracks by saying "next" (at home). I purposely look aways just to see the screen dim and look on it to see it turn on. Sure, gimmicks, but I had underestimated how much they enhance the fun experience.
Today I had S Voice on most of the time to do things for me, such as search for places and manage calls and messages.
The hardware is absolutely top notch too, everything inside of this phone.
I feel like people complain about the S4 because of all the software gimmick, I did too, as with S3 they weren't that obvious but having the S4 I see them very differently. They actually work better. What Samsung aims at is making a phone that would be fun to use in that cyberpunk kinda way - like in the movies where you have technology knowing what you want to do without you having to even peep at the UI. It's like a first big step in that direction and even though I'm a geek and I understand how they work and they are gimmicks, having the full package of them really is fun to use. I hope Samsung don't stop perfecting them because of all the complaints.
I totally recommend the S4 right now. Right now there's probably nothing that comes close to giving this great overall experience. If you like the underdog you can go for the HTC One but you wouldn't be as happy with it, you wouldn't know what you're missing and it'd be a good phone to have too, of course. Just not as technically cutting edge and entertaining.