Undertale is a very good game, but I will warn you right now that it's overrated. People seem to be praising Undertale like it is the Citizen Kane of video games. If you are thinking about playing Undertale, it's best if you forget about the hype surrounding this game. Just try to enjoy the game on its own merits.
Before anyone says anything... "overrated" does not equal "bad", but I need to make it clear that Undertale will (at least slightly) disappoint anyone who goes into it expecting the greatest game ever. I still love this game, but it certainly isn't my top favorite video game of all time.