I have to admit up front that I'm a big fan of the Samsung phones, so keep in mind that I do have a bit of bias here.
That said, for the most part, I love my S6. For the sake of brevity (as best I can do), here are my complaints and/or suggestions about it:
1. Battery life is not horrible, but not that great either. I am lucky in that I typically have access to a wireless or wired charger, so it's rare that I worry about battery life. However, it is kind of annoying that I need to constantly check the battery to see if I need to recharge the device. I said early on when I first got the phone that I don't understand the whole industry drive to make the phones as thin as possible. They are PLENTY thin, and having made the phone as thick as it needs to be so that the camera doesn't protrude from the back would have been the right choice - as it would have eliminated that issue, as well as given a lot more room for a larger battery. Two birds with one stone, without sacrificing the quality of the phone in any meaningful way.
2. Not sure if this is more of a Samsung issue or an ATT issue, but together they both put a fair amount of "junk" on the phone that really doesn't need to be there (ie: useless apps and features). I don't know enough about how all this works to evaluate if this causes the phone to be slower, or if these things eat into battery life ... but I do know that I basically turn all that stuff off from day one ... disable apps, turn off features, etc.
3. Lastly, again not sure of the tech side of my comment ... but I wouldn't have made the back of the phone out of glass. I run a very slim case ONLY to protect the back of the phone so that I don't feel the need to completely baby is when setting it down, for fear of scratching/breaking the glass back. Some have said that glass was used so that the phone could do wireless charging w/out a special case. I have no clue if this is true, nor do I know if the new S7's are glass-backed or metal. If metal would work with the wireless charging, I think it would have been a better move to make the back of the S6 out of metal.
So, there you have it ... I'd buy the phone again as the pros outweigh the cons for ME ... but if I were someone who was "out and about" to the point that charging whenever I wanted to was not an option, I may have to reconsider that and choose a different phone.