Undertale is amazing. It's a funny, whimsical, sometimes nonsensical, sometimes disturbing turn-based RPG with a flair that I cannot help but compare with Earthbound in some ways. The strength of its plot, the complex story you can experience multiple times, the depth of it's mechanics (pretty cool wario-ware battle system) and the humor make this a game you won't want to skip if you're a fan of any of these things.
I have nothing but praise for Undertale, not a single bold line in this whole review, I didn't find anything wrong with the game on my first playthrough and now I've started my second one because I want to see another way to do things. Just the fact that the option is there is really cool. Otherwise, on its face, you have a typical RPG. You walk around, talking to NPCs, having random encounters and equipping items, always moving forwards to advance the plot.
As you go, the mechanics grow in depth too, colors start to become an important part of the battle system. Enemies become much more difficult and you have to master the attack and defense systems to make it, moving your heart cursor around to dodge projectiles. The end of the game also freaked me out in a good way, and I really enjoyed what it did. Of course I'm not going to talk about it in detail here, because it's much of a spoiler.
And that's pretty much true for a lot of the game. The humor is really good, but repeating all the good jokes here would break their punch when you experience it. That being said, I feel that playing the game a different way now that I've completed it will let me see more content from familiar characters and situations. The music is great, the art is great, the writing is great, what else can I say?
You really should play Undertale, I can already see it going on my 2015 GOTY list and I can see myself replaying it at least once - well, I guess there are only two ways to play it, right?