I beat this game a while back. It was a weird game for me... I'll start by saying that the game, in almost every aspect, is really well-polished and well-done. The battle system was extremely entertaining, as literally every single battle you fight is drastically different. There are dozens of different paths the game can go, with three major endings that you can get, which are based around how many things, if any, you killed in your playthrough.
However, with that said, I didn't really enjoy it that much. The game's humor, like Chpexo wrote, didn't really strike me. I felt that the game was trying too hard to be "weird" and "random" and "funny," that it often felt pretentious. The contrast between the goofy surface versus the intended gritty and dark story made me uncomfortable. So many things in this game were just props to showcase how quirky this game can be. It felt slow, especially toward the end. I got the bad-ish ending, and it was anticlimactic. All of these things just turned me off to an otherwise great game.
I will definitely give Undertale another shot, though, and try to get a better ending.