I'll keep it straight-forward. This movie is phenomenal. The writing was well-done, the characters were real in the sense that not everything is perfect, and we get a main character that we start off loving only to have mixed feelings for towards the end.
Everything becomes a mess halfway through, it's thought-provoking themes make the movie well worthwhile if you're open to the idea of asking why characters act certain ways. I'm sorry that this review seems vague but I wouldn't want to influence any new watchers perspective or spoil any of the movie. World's Greatest Dad is not for everybody but if you do watch it, asking questions and solving them for yourself might make the movie become that much more interesting.