I won't spoil any of the story here, for anyone who has not played it, but I wanted to put my thoughts in order on this particular game.
I managed to avoid most of the hype surrounding this game until I was able to finish playing it. So I'll try not to give it any more, since I tend to find that hype ruins almost everything. I will say this about it though: I haven't played an RPG completely through to the end in a long time, probably in about half a decade. In fact, I haven't finished very many video games at all in recent years. There just hasn't been anything that caught my attention and compelled me to continue until the end, with the exception of the Dark/Demon's Soul series.
That being said, I finished Undertale, and I did it without feeling like it was a chore. It reminded me of the reason I enjoyed RPGs back in the day (NES, SNES, PSX), and I'm so glad of that. Simply put, it made me smile, genuinely and truly. If you are reading this now, and are a fan of the old style RPGs, then I recommend it, assuming you have not yet played it.