I have to say, after finding my S2 to be absolutely brilliant, my S4 has been a bit 'meh', solid featureset but not outstanding and much more niggly problems (now the bottom of the screen flickers and none of the supposed fixes i've found fix it).
Since trying a Moto G with near enough plain Android on it, i'm finding myself looking away from Samsung for the first time in years and not much of what i'm seeing of the S6 is making me think I need to have one on release, unlike the S4 which I got during the release week :/