I recently bought the soundtrack to this game; it's a mixture of 8-bit chiptunes, SNES style music, and more modern instrumental stuff.
The game is rather unique. Most role playing games are centered around the concept of killing monsters and gaining experience to make your character stronger. It is nice to see a game that can be completed without simulated or implied violence. It's not that fighting monsters or enemies in games is necessarily bad; it's just nice to have something break the mold a bit. Undertale actually discourages killing the monsters through the way its story plays out.
Still, what stands out the most in the game is the creative soundtrack. Toby Fox does one heck of a job with this.
(On a side note, he's worked on music for the web comic "Homestuck", which, for some weird reason, I have avoided reading.)