Before I begin I wasn't even aware of this game until about October of 2015 when I saw a post about "sexy skeletons" on Tumblr. The last skeleton on the list was the skeleton I soon learned was papyrus though I thought at the time he looked like a rip off of Sir Daniel Fortesque from the medievil games but to be honest I mistook Lewis from the Mystery Skulls Ghost music video to be the same thing so I decided not to judge just yet.
Well as Tumblr is known for my dash soon became assaulted with fanart for the game and I wound up blacklisting it because it begun to get on my nerves though a lot of it got through my blacklisting due to people not tagging the blasted thing.
Well soon I began to learn more about the games and I even did a bit of research for it on the games wiki (to be honest the characters all reminded me of better characters from other media plus it looked like a rip off of Earthbound only with skeletons and goat people) about the characters and stuff and also from talking to a friend who likes the game. I soon learned two things.
1: a lot of the steven universe fans had moved over to the games after some drama involving an artist who made fanart that wasn't "acceptable" to the fanbase. and 2: these fans happened to be very obnoxious social justice warriors.
I being sceptical of this let it alone for a bit longer before I finally discovered the truth of this matter as well and also making the discovery that a chunk of the fanbase was full of very angry people who acted like toddlers when someone said they didn't like the game.
So after all that I came to my own conclusions and made up my own mind.
This is what I concluded.
The game isn't worth playing. The graphics remind me of something you'd find on the Atari system or the very first Nintendo which is pretty laughable with how far things have come gaming wise since the 1980s. The characters if you can call them that are nothing special. They're boring and annoying and are not appealing to me at all. They look like rip offs of characters from better games. Many things the "sans" character (who in my opinion has a face I want to beat in with a baseball bat)says comes from other media like South Park. A lot of the music is ripped off of the Homestuck soundtracks which is just a reminder that I could be spending my time doing much better things. The game itself has only three ending options (this is not too much of a problem since there are games with a good bad and middle ending though these games also have a fourth TRUE ending ) and you can get them all by only making minor changes in gameplay. since if you kill one character you get the middle ending and if you kill nobody you get the "good ending" which IMO is kind of unrealistic and boring.
As for the fanbase. There are good fans who like the game and don't act like jerks about it but they are usually outnumbered (vocal wise) by the VERY obnoxious superfans who treat the game like the second coming of christ (it's a figure of speech and I don't care if you don't believe in god) and if you play it "wrong" or say anything bad about it they will scream, moan and attack you claiming you need to play the game to understand it's supposed "greatness" and if you do play it they will get smug and high and mighty about it and yell at you for playing it "wrong". To them the only ending anyone is allowed to have is the good sunshine and rainbows friendship is magic ending where the homicidal flower is touched by your twilight sparkle attitude and become a wonderful and happy brony.
There's also the fact they're a pack of social justice warriors who will blow a gasket if you "misgender" the main character. This might also explain why Mettaton has so many fans because that robot's "true" form (which looks like something out of the rocky horror picture show) is a SJWs dream come true. Say anything bad about these things or even anything else in the games and you are transphobic homophobic, racist, sexistand every other SJW bad thing out there. FYI fans Dr frankenfurter did it better.
The fans who are maniacs also seem to have a major problem with the FNAF fandom and I don't know how the feud started but I am aware they went and send death threats to the creator of that series which is really uncalled for. I myself don't like FNAF but I don't go around acting like a jerk to those who do. To be honest things I don't like I tend to ignore and avoid even if it's shoved down my throat. I might join an anti group for the things I don't like but that's about all I do. I don't go attacking people who like things I don't. I continue my life liking what I like and ignoring what I don't and I don't attack a fan of something unless of course they come to ME and act like a dick to me for not liking things.
Conclusion: I don't care for undertale and I am not interested in playing it. The graphics are bad and the characters and plot are dull and unoriginal. The bad part of the fandom is a pile of hypocritical jerks who are worse than the Hetalia and My Little Pony fandom combined and I have no desire to be part of it.
My advice? If you want to play the game go right ahead but if you become a fan don't act like a dick about it especially to people who aren't interested in it and don't want to be. NOBODY likes a pushy fan.
One last thing: the creator himself has said he doens't mind if people hate the game because he, unlike his game's fanbase, knows people have different interests and tastes. Maybe the Undertale fanbase should be more like the game's creator. We'd all be happier for it.