Maybe it isn't when you have the manual at hand, but its nearly impossible to complete the game for the first time without looking for answers online. Especially if Fallout is the first isometric rpg you play. This game does not hold your hand. I remember struggling as hell when I first started playing the game, and I nearly gave up, like many other people have. I think its Fallout's extreme difficulty that kept bringing me back to it. For me the extreme harshness of the game's mechanics and UI reflected the harshness of the world it was depicting. I felt like I was there, struggling to survive in an unforgiving post-nuclear wasteland. Alone, without anyone to guide me, comfort me throughout my journey.
I never felt more immersed in any piece of fiction than in Fallout.