I'm with you. Entertaining enough, but badly written and way too long.

I saw someone mention that it's a book that feels very specifically Reddit-y in its humor, and I think that largely explains the popularity here. To boot, it's a very easy read with absolutely nothing going on below the surface (ie, it's very straightforward when it comes to the main character's emotions, which are not especially complex). And, of course, there's the fact that it fulfills the perfect fantasy of the main character >!saving the entire world and then making a huge selfless "sacrifice"—which is actually anything but—to also save his friend!<.