I've been using my S8+ quite a bit since it arrived on Wednesday. I am coming from an S7 Edge and had the Note 7 before that. In general I prefer bigger phones, but I was happy that Samsung chose to keep the width of the phone as small as possible while growing the length in order to accommodate a larger screen.
HOWEVER, I am experience some major hand fatigue while holding this phone. After using it for several hours, when I pick up my S7 Edge, again, it feels much more comfortable and "lighter" in the hand.
I've realized something. Even though the S8+ only weighs slightly more than the s7 Edge, the length of the phone really impacts the "feel" of the phone in terms of how heavy it feels. That is because I am gripping the phone along the lower half of the body, and I generally hold the phone at a 45 degree to 60 degree angle as I am using it. The portion of the phone that is positioned above my hand is more "impacted" by the pull of gravity than a shorter phone. I would compare it to a lever and fulcrum, and I'm feeling the impact of a longer "load arm" of the lever... If that makes any sense.