I just experienced the first hour or something of Undertale, and I really liked it.
Some of the graphics sometimes are a little boring, but everything else is just awesome, and I smiled from beginning to end. All the characters are just so funny, and full of personality, and I liked all of them (except for the flower guy at the beginning, it's such a jerk ). The two skelly brothers are the best. Papyrus (if I recall correctly that's his name) is a very comic character, who tries to seem all imposing and such, but fails every time, and Sans (again, I hope it's his name, I really haven't paid a lot of attention to the names unfortunately) is just great (I laughed so hard when he told the protagonist to "hide behind that very conveniently shaped lamp" ). When I had to leave the ruins, it was a very emotional moment, especially at the end of the fight with Toriel. I personally decided to choose a "pacifist" route, so I haven't killed anybody.
MORE STUFF (EDIT): oh wow, I was able to beat Undyne the first time, but it was soooo challenging. Those arrows are deadly. I managed to survive the final wave with just one single life thingy, I loved it!
By the way, I loved when she was on top of that cliff and she wanted to tell the story of the kingdom and then she was like "well screw it, you're gonna die anyway!", I laughed so hard