Nice game. Its main contribution is the moral choice mechanic. I ended up using "mercy" all the time and got the best possible ending. The game's creator Toby Fox is also a composer for Homestuck and the soundtrack has a very Homestucky feel. Also, some of the creature designs are pretty cute.

The bad: the story is paint by numbers, the writing is hammy, the characters are cardboard, and the art is average pixel art fare. The gameplay challenges are either boring puzzles or bullet hell, neither of which are very relevant to the story. There's some humor but it didn't make me laugh out loud. Everything works together just enough to make the moral choice seem important, but not more.

However, from a certain point of view these drawbacks don't matter. My theory about art says that successful art needs to be new and enriching to viewers in at least some aspect, and repeating an existing idea doesn't count as art anymore because you can't enrich people in the same way twice. By that yardstick, Undertale succeeds as art where most other video games fail, even though they might be better in execution.