I find nothing about Undertale special or worthwhile in the slightest, and while much of that is certainly personal taste (I am a Halo guy, RPG's don't suit me more often then not) And I just think the entire thing is amazingly overblown. The music is memorable, sure, but not in a good way. My younger brother enjoys blaring the soundtrack as loud as possible, and instead of rocking out it makes me want to go back in time and snap Toby Fox's fingers off his hands.
The whole thing just feels like Toby Fox was really aiming to snag the Fnaf fans and people like them, that group of fad-following kids who don't seem to know better. He seems to have carefully designed the game to capture popularity and have a lasting fanbase, like the whole thing was made by reading off a chart and just launching whatever he ends up with.
Don't even get me started on the stupidity of Monsters and determination.