The fanbase is cancer, the story is beyond stupid and makes you feel justified in committing genocide (especially because the only good final boss that's not just a fucking cutscene pretending to be a boss is Sans), the music is really good, and I really like the combat system. Playing it at 1.5x or 2x speed is also really fucking fun because it makes the combat manic as fuck.
My biggest complaint beyond the fart huffing "treat this GAME like you're actually hurting people" story is two of the three final bosses. After all the fun the combat system of this game offers, you get these two fucking wastes of time and your accumulated skills.
Asriel can go die in a ditch, shitty character all around and he isn't even a boss fight, he is a glorified cutscene with no loss state. Sure, if you "die" in the fight you have to make it through a few turns to start getting the script going again, but you never lose overall progress no matter how many times you "die". All those extra hours of work because you're berated for being "evil" otherwise and you get this faggot shit of a boss. Wooo.
Omega Flowey is pretty much the same, it gives you the illusion that it's a boss fight, but it really isn't. This "fight" works differently from Asriel, but again, you don't really lose progress, when you load back in, if you made enough progress last time to hit a flag, it persists. This is supposed to be a final boss, Toby, a CHALLENGE not a cutscene you cunt.
In summation: Overall it's an enjoyable game, but it gets waaaay too up it's own ass at times especially because the story is just too black and white to be as preachy as it is. Avoid its rabid fanbase of retards like the plague though. I do like the whole "saving" concept and the more meta stuff in the game, they're interesting and have definitely given me inspiration of how to hide some secrets in unique ways in my own projects. It's fun, but it's no 10/10. I will give credit where's due though, it's very impressive for being almost entirely a 1 man project, I can't deny that Toby has quite a lot of talent.