The only real con I see about the s6 is the battery life. I keep hearing that it's not very good at all, and not to expect the device to last all day under medium to heavy use. However, as was pointed out, the fast charging and Qi charging could overcome that flaw. I don't know any places (or at least frequent them enough) to know where I can put my dying s6 on a Qi charger though. Though I suppose if my s4 lasts me just about all day right now, I wouldn't have a problem with an s6.
I did read that Samsung is aware the battery life is an issue and that they could be rolling out an OTA to fix some bugs to improve battery life.
I was bummed that Samsung caved in to all of those "expert" bloggers out there and made a unibody phone. I consider myself a power user and believe that there will never be a phone that can win in battery life versus me.
Since the inception of fast charging and small package per mAH portable batteries it is no longer a concern. I originally purchased a new extended battery pack for my i6 Plus, but found myself also using it for my Note Edge. It's so convenient that I never purchased extra batteries for the Edge.
I'm now awaiting the Note 5 Edge with presumably all of the "downgrades" that people have perceived the S6 to have gotten.
Samsung nailed it with this phone. The only thing I hope they continue to strive for is to cut down on the included software that is preloaded in the phone.