I've had the S5 for several months now, before it, I had the S3 for several years.
While the S5 has many nice features, I'm sorry I bought it and wouldn't buy it again.
My complaints have to do with the notification system and the phone making unrequested changes while in my pocket.
For example, I need to constantly check the volume level on my phone. Many times a day I'll hear the waterfall sound with the phone in my pocket and I'll find the phone has turned the volume all the way down.
Pocket dialing is frequent since once a notification comes in the screen lights and when that happens, random menu selections take place.
I've called ATT customer car about this and their response is that they have gotten numerous complaints but there is no workaround.
I suppose if you work in an office and the phone spends the day on a desk the problem is less significant but spend any time in the car and these issues will occur every time.
If I didn't have this problem I'd be very happy with the S5 but as it is, its a show stopper for me and I'm sorry I bought it. I won't buy a Samsung phone again without verifying this problem is solved. My wife is ready for a new phone but she's written off the S5 after seeing how annoying this problem can be. Of course, YMMV.