Undertale is as much a story book as it is a game. Most of its fun comes from playing with and subverting your expectations. And the fact that the game actually makes you own up to what you've done during the game.
I enjoyed the Bullet Hell aspect, because I happen to be a Touhou fan as well.
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And looking around here, I think I'm rather disconnected from the Undertale fanbase. While the skeleton brothers are indeed my favourite part of the game, they are nowhere near the main highlights of the game for me.
They are a just a *part* of the game's quirky and memorable cast.
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I'll admit that I don't like hearing people bash the game without having played it, but I understand. That said, I don't really agree with judging a game or show from the fanbase though.
/endrant