Here's an anecdote about battery life that may or may not mean anything.
I have had three S6 units. The first was returned because they shipped me the 32gb instead of the 128gb I ordered. The second was returned because of splotches under the screen. The third is the one I'm using now.
The first unit was giving me 2.5 hours of SOT and 9 hours of total battery life according to GSAM. The second was 3 hours SOT and 9.5 hours of total battery life according to GSAM. I disabled location services and a bunch of Google and Samsung apps on the second unit and got an incremental improvement. In both instances, the one thing that really stuck out was the Wifi battery usage -- it was frequently higher than the battery used by the screen. If it was wasn't actually higher, it was close. Sometimes it was over 50% higher.
Then I got the third one. I decided to see if Facebook was the culprit behind the wifi killing the battery. Instead of logging into the app, I manually browsed to the web version on Chrome. The result was significant -- after a week, SOT was over 4 hours, and total battery was 13.5 hours. Screen was the biggest battery hog once again. On a couple of occasions, I had crazy numbers like 1.5 hours SOT with 85% battery left. Friggin' Facebook! All this time it was Facebook! And then, after a good week of using web only, I re-enabled it.
So it turns out Facebook did have an impact, but nothing earth shattering. I'm now down to 3:41 SOT, but oddly enough, my total battery life jumped up to 14:39. This is with location services enabled and set to accurate and no bloatware disabled. So what's the difference?
Taking into account this anecdotal evidence coupled with the fact that most of the terrible battery experiences are from the first wave of T-Mobile units, my conclusion is that the first batch had bad antennas. Bad enough to kill the battery in under 10 hours from standby alone. The second unit I had reported the Sony sensor. My current one does not, which indicates that it's of newer manufacture.
Considering battery life is literally the sole complaint with the S6 (well, once you wade past the "extSD and removable battery" comments) and a big chunk of this seems to be improved in the newer builds, the S6 has no real weaknesses (again, once you wade past the "extSD and removable battery" comments). There are plenty of people getting good battery life from the S6, and at this point probably more people getting good battery life than bad. Personally, my S6 is getting about the same, perhaps a skosh better battery life than my S5 did (I have Exchange active Sync enabled for work, and it's a real battery killer no matter what). I've had two occasions where I left the house all day away from an outlet, and although I did use an external battery pack to recharge in the middle, I could have made it back with battery to spare both times.
My verdict: the S6 is a superior device with an industry leading camera (for me, Auto HDR gives the S6 camera the edge over the G4). I wouldn't let the battery be the sticking point to your purchase.