The best way I could describe the game is "cute". I never managed to get to the end because for some reason this game always made me sleepy after a while, but it's definitely recommended for those that like quirky jrpgs like Earthbound and Mother 3.
I played about 45 minutes of the demo, and it reminded me more of games like Candy Box 2 or Frog Fractions than Earthbound. IE, games that are intentionally counter-intuitive (I guess I Wanna Be the Guy is another one, though a major focus of that was also difficulty). Earthbound was humorous, but played more or less like a straightforward JRPG from what I remember, and had a fairly coherent setting.
But Candy Box 2 and Frog Fractions were a lot better, and they're free (and since I stumbled on both by accident, I imagine there are many more randomly different free games out there). The humor in those games is also much better. Undertale's humor includes things like giving the PC, who's a child, the option to flirt with your human-cow-mother thing (and you get to go on a date with a skeleton later, apparently).
The game play seemed to consist of entering a room, figuring out the gimmick, then going to the next room. At first I tried to do a lot of counter-intuitive things, since I kept hearing about how great the world interaction and reactivity was. But there really wasn't much to do and not a lot of ways to interact with things. The game seemed to also be completely linear (everyone who plays it will go to Room A, solve the puzzle there, then go to Room B, solve the puzzle there, then go to Room C, solve the puzzle there, etc.).
It's punctuated by extremely repetitive combat (though some have said that the encounter rate in the demo is higher, so that might be part of it), which is part of the reason why I didn't bother playing to the end of the demo.
I heard a lot about the reactivity, so I decided to check the FAQ and see what I'm missing. If I kill a certain monster the character I see afterwards will criticize me for killing it; but if I spare it, it criticizes me for sparing it. There seems to be lots of stuff like this:
The next area will look different depending on whether you petted Lesser Dog a lot or not. If you only petted him once, you'll see the dog making snow-dogs. If you petted him a bunch, you'll see long, deformed snog-dogs all over the place.
Well, at least it has multiple endings, right? Huh, here it says it only has 3. I guess that's OK. Oh wait, the genocide ending is about hours of mind numbing repetitive grinding ("It's supposed to be boring!"). Well, it has 2 at least. Oh, the pacifist ending can't be obtained the first time you play the game? Eh, so most are going to be stuck with one ending on their initial playthrough. Eh.
Reading the FAQs, It seems that a lot of the "OMG can't tell you the spoiler!" stuff is about things that happen when you replay the game. Kill someone in one playthrough, and they comment about it in the next, etc. I guess that's kind of interesting? But would I want to keep playing the game when the game itself isn't terribly interesting and I'm doing the same exact thing 95% of the time? It also seems that the previous comment about people not wanting to spoil things is more about not having a good example. "Play this game because if you kill a guy on a playthrough and then talk to him on the next playthrough he'll say he remembers you killing him" - not exactly an argument that makes one want to run out and buy the game.
After the high praise this game was getting I was expecting, if not a great game, at least something somewhat noteworthy (once you got past the graphics/questionable humor). Now I'm wondering if I can blacklist some of the members here so I never take their suggestions seriously again.