I just bought the Samsung S6 yesterday, and after holding it for about 30 minutes my fingertips started to feel strange. The next morning they still feel strange, like numb, or after they have been very cold in winter time. I am just scared to touch the phone again. It's seems like a kind of magnetic field on the back side when the WIFI is on and you hold your fingers on the back and writing with your thumbs. I have never had any problems with phones of similar sizes or weight.
However I have the same thing when using the integrated mouse pad from the Samsung SensS R460 notebook. So I always use external keybord and mouse.
I am very disappointed as I just traded in my Iphone 6 as it the screen was not working properly, in the phone shops in Cambodia they do not give warranty. So with the left over money I could still purchase the Samsung S6, very beautiful and easy to use, however now I go back to my old phone and have to trade in the Samsung S6. There should be some research on the level of electric fields its create. My fingers might be a bit sensitive but I just cannot touch it anymore. But no problem with earlier models from Samsung.