If Jumper is something of a standard for insanely hard but still fair platformer games, then IWBTG is the same for unfair platformer games. What do I mean with unfair? Unfair means it’s not obvious from looking at a screen what’ll kill you.
Yet, unlike other games, in IWBTG it’s not because of bad game design, it’s part of the design. IWBTG is made to punish you. I can imagine the creator sitting before his computer designing these levels with a mean grin on his face. It’s sadistic, but strangely also very entertaining at the same time.
If you know your gaming history, you’ll have many nostalgic flashbacks, for example incorporating Tetris or Arkanoid (or Breakout if you’re even older) in a platformer strikes me as a touch of genius. Or the boss sprites.
Still, this game is hard, first you’ll have to learn every screen like a memory game, then you’ll have to jump precisely and well. Or you can watch the playthroughs on YouTube.