I heard of Undertale gradually throughout internet references and happening upon the OST on Youtube. The OST is excellent and was a major factor in me deciding to play the game.
By the time I decided to play it, I was familiar with the basic idea: there are humans and monsters, killing is always optional, and there are Pacifist, Genocide, and Neutral routes. I'd seen some gameplay and was familiar with most of the core characters' names, as well as a couple unfortunate spoilers about each from scrolling too far down in the comments on the OST videos.
Although what I'd heard about it made me worried about an ideological pacifist slant, I was interested in the idea of a story that revolved around such choices. I thought that concept had a lot of potential, and that it was less likely to be handled poorly if it was at the center of the game. And I knew the game had an excellent reputation and wanted to be in on the references and all. So I gave it the benefit of the doubt and forked over $10.
I'm surprised to be saying this as I went into it expecting the primary appeal to be narrative, but the combat system is solid too.
The main mechanic is bullet hell-style dodging during enemy turns. There's no unavoidable damage (or almost none; there's one enemy that might be an exception), and each enemy has a good variety of moves. Just about all of them can be evaded without prior knowledge.
One thing that's done really well is how their attack patterns progress throughout the fight, especially bosses. Bosses usually start with their easiest attacks and break out new ones or more sophisticated versions of early ones as they go on.
They also introduce a few different color-coded types of attacks and modified states for your avatar that change the dodging rules, massively expanding their design space beyond simple messes of bullets.
Your own attacks are modulated by a "watch this beam move and press it close to the middle" system, but to stop it from getting too old, weapons you find later make it slightly more advanced, like giving you multiple beams to peg and sending them with randomized spacing.
It's a silent protagonist game. So much for role-playing! I can't express how much this harms the potential of a story with intricate lore and lots of conflict resolution for the player character to have no dialogue in most situations. As with Dark Souls, it's often used to prevent the player from asking questions that any reasonable person would ask and that would presumably get an informative answer. The player's interactions with Undyne are harmed the most.
It's not fatal though. Unlike Dark Souls, Undertale's major NPCs play meaningful roles in the story and have significant depth (for which you don't have to 100%, cross-reference every item description, read game files and headcanonize into conclusions that have almost no basis in the actual game, which is how lore seems to work in Dark Souls).
The game revolves around your decisions whether to spare monsters that fight you. I was disappointed, though not very surprised, that this doesn't involve any real exploration of that possibility space. There's no dialogue where you try to actually reason with the monsters; you can never ask anyone why they're trying to kill you, you just solve their little ad-hoc conversation puzzle (see below). And most of the random encounter monsters are too impersonal for said conversation puzzles to feel like anything other than a boring minigame. Their designs and names tend to be jokes that all their dialogue revolves around. You're told in some conditional dialogue that "each of those monsters could be someone else's [close friend]", but that notion carries very little power when the random encounter monsters have such un-person-like, ad-hoc psychology and never get another word after you spare them.
Of course, changing this would've been a massive overhaul because it would've required thinking up a way to prevent exposition from getting out too soon.
You find out why the monsters are trying to kill you halfway through, and it's a pretty damn good reason, but even after that you can't try to reason with them. In the case of the boss who more or less explains it to you, the only way to spare her is fleeing on a specific turn and then she just stops trying to kill you after that, even though there's no reason that should change anything that refusing to attack her doesn't.
Other than that though, all the main characters are solid, nuanced and have real development. Even characters I'd previously planned to kill, I ended up finding quite likable and their actions much more defensible in hindsight.
Honestly, my opinion of the game went from 5/10 to 9/10 just over the period from the end of the final boss to the end of the game on my first (Neutral) run. I can't even talk about it much without ruining it, but I guess a spoiler tag would do: it involves meta stuff in the style of Doki Doki Literature Club, and an incredibly satisfying return of someone from the beginning. No other game has made me feel so lucky to be able to try again.
Both the Pacifist and Genocide endings hold mind-blowing reveals that the other doesn't. The Pacifist ending is absolutely glorious. It involves all the characters, and has an epilogue about as good as Lost Odyssey's.
I'm awed by how much work is put into the many different Neutral endings. I only saw one (Papyrus's), but I read about the possibilities on the wiki and... wow.
One thing I'm disappointed about is that the game doesn't lend any serious consideration to the idea that it is justified for the monsters to kill you to free them all. There's not even an ending where you voluntarily submit to this, even though that's the most logical way for this to end if the protagonist has good intentions (and was the ending I thought I was going for on my first run).
Of course, I have to make the monarchy criticism. It normalizes monarchy by having both a king and a queen as designated goodguys, like all too much fantasy fiction. It also normalizes schools. But the enforcement of these institutions is never seen or mentioned, leaving it possible that the schools aren't coercive and the monarchic titles are misnomers, which I guess makes it still an improvement over the par for this world's fiction?
Combat conversation system is capricious¶
In general, getting enemies to be spareable involves the Act menu dialogue options, which let you (forgo an attack to) make some attempt at conversation or other non-violent action during a fight. Unfortunately, there's often no predictable logic to what options will work. I recommend asking the wiki if you want to spare an enemy and don't feel like guessing in the dark. I used the wiki heavily during my Pacifist run and never regretted it. Each character has a separate "In Battle" page, so it's not too hard to do so without spoiling stuff.
There's also the Flee command; I never figured out exactly how it works. Early on it seemed to be 100% reliable, but it's not; then I thought for some time you could only flee against a single enemy, but that's not it either; I saw exceptions on both sides. I couldn't find anything on it on Google either. I think it just has a high chance to work. The main downside for a Pacifist run is that fleeing doesn't give you gold either, while sparing enemies without fleeing still sometimes gets you gold.
Undertale is denser with laugh-out-loud moments than any other game I've seen. Pretty much every line from Sans or Papyrus is gold, and the friendship scenes and most of the Mettaton encounters are Backstroke of the West-level.
The overworld consists largely of puzzles, most of which aren't too difficult, though there were two I looked up: the two where there's no explicit puzzle but proceeding involves pressing the action button with no obvious target. (To be fair, one of the two seems legitimate to me in retrospect.)
The game is significantly harmed by the lack of a fast travel and the slow movement. It makes it very unappealing to revisit previous areas, which is especially bad since that's required for a lot of important scenes.
Verdict: it's excellent and you should play it.