I'm another very satisfied AT&T S6 user ... superb camera w/ the Sony sensor ... 64gb on-board stoarge ... flawless screen/glass/bezel/etc ... no battery life issues ... quick-charge works amazing ... no lag (using TW or Nova Prime) ... no dropped calls ... yadda yadda yadda.
I came from an AT&T S4, which worked perfectly as well, except for the built-in lag, and the fact that as it's gotten older (read: more software, updated software, etc.) it's gotten a little slower over time ... but functionally, the phone was flawless when I switched to the S6.
All the complaints I'm reading about the S6 (well, maybe not complaints, but "issues") ... I find it hard to believe that it's the fault of the S6 with the exception of a few random hardware units that are out there. Seems that a lot of this stuff is the newness of the phone, and the Lollipop OS and all the carriers getting things ironed out. I mean, the phone just hit the market. I know they do testing etc prior to release, but it always takes time once the phones are out there in droves so that people can find other flaws, bugs, etc. that need to be ironed out.
It really sucks for those who have issues, but I would just say "Be patient and let "them" work out the issues so that you can truly enjoy your new phone.".