My thoughts:
Photos look great - for the most part. If you're used to another platform or phone, it just takes a little while to learn what it can do and how to use it well - Like any other equipment, it's the photographer who makes all the difference.
Video performance in low light and concert lighting isn't great, but when I can bring in something bulkier than my phone, I use something better. It's a phone. I'm not shooting footage for a documentary to be released on blu-ray or some such thing.
Audio performance during a concert, however, is much worse than I expected. I'm coming from an iPhone 4s to the S5. It's a world of difference and I'm glad I made the change, but the iPhone just managed to automagically capture and process great sound at any level. The S5 sounds like 10 wool blankets over the mic. No highs. Distorted lows. C'mon, Samsung can't copy that?
(I kid, I kid...)
So, here's a video I took Friday at a concert. It's the only of the several clips I took that I bothered to upload because it's the best sounding. I moved a little further from the speaker for this one. Distortion kicks in at 3 - 4 minutes when it gets loud. At 7 minutes, it's abysmal.