This game is fantastic.
I can wholeheartedly recommend it for its story alone. The way it messes with you and the way your choices throughout the game has meaningful impact - it really makes for an unforgettable experience. The characters are goofy and nicely characterized. It has its fair share of twists and it really takes advantage of player's expectations from what you'd consider a run of the mill RPG - it is VERY good at screwing with you. XD; Which is NOT a bad thing at all considering the fact that predictability in games is one thing most people dislike.
Art may be a sour point for some but it really works for the game. Minimalist design is a choice they took up and it really helps with the game's personality. They made sure there's a decent amount of animation where it is REALLY required - you being patted on head by Toriel, Sans' jokes and that accompanying zoom-in and sound effect, papyrus' constant poses and whatnot. All make for subtle details that really add to the overall package.
Combat is great. You can even avoid fighting ANYONE till the end if you want. I killed a few people during my first playthrough but I don't regret that since the combat system is very fun. Attacking is a qte based event which tests your timing. The best part about the combat is definitely defending, though. Since all enemies use varied kinds of attacks on you, the defending turn can change from anywhere between you dodging easy-to-dodge puppy attacks to the hardest bullet hell (something you'd find in a Touhou game or the like...). And if you choose not to fight, you can talk, flirt or do any of the bazillions of the enemy exclusive "acts" possible. REALLY makes for an amazing gameplay, trust me.
And the music. Oh my god. I've been listening to Mr. "you are gonna have a bad time"'s boss theme on loop for almost an hour now. It is addictive, to say the least. All the other soundtracks utilized are fantastic too. Almost all boss tracks are great. And the rest of the tracks are very personality-rich too.
What REALLY makes this unforgettable, though, is the way it messes with you as a player, its self-aware nature of it being a game and the way it reacts to your choices in the game. Each action unlocks some new dialogue, some more new choices and may even lead to you having anything from an okay to a "very bad time". Simply put, the amount of effort that's been put into this game to make it worthy of multiple playthroughs is absolutely staggering. I can't even. Just wow.
I'm playing through it a second time. This time I won't kill a single soul and finish the game on LV1. With no XP gain and no LV gain, I'll have a tough time, sure. But I'll still do it because the result, I know, will be worth it.
Then, just after that playthrough, I plan to go on a run where I kill EVERYBODY. I don't think my heart will be able to handle it after a peaceful run but...I wanna try it anyway. To see how morbid and depressing this game can get. It already made me somewhat depressed in my "neutral/normal" playthrough (and I hadn't even killed many people ;____;) so I'm sure it'll be painful. But yeah.
ANYWAY. tl;dr - game is fantastic and you're missing out on one hell of an experience if you miss it.