From what I hear, many people like Undertale for its witty writing rather than its gameplay (and also because its a lot like Earthbound). The game's writing is basically like Japan's visual novel genre where you're decisions affect the dialogue ever so slightly and effect which ending you get. If you hate a lot of story and play RPGs for their gameplay, this game is not for you. You only have one party member to my knowledge and this makes the battling monotonous when the bullet hell segments lose their novelty. Of course this is my experience.
I was initially dissuaded from buying the game because the bullet hell battling system seemed like it didn't really fit, but after watching a friend play the game, I decided to buy it and play through it myself. It turned out that the battle system had significantly more depth to it. It's not just bullet hell as the battle system changes very significantly depending on the enemy you're fighting. There are platforming, shmup, and rhythm segments as well. The battle system is also used for narrative purposes during some fights.
So I disagree, the gameplay is fantastic and the battle mechanics never lost their novelty on my first play through.The gameplay is definitely different though from your average RPG where the "gameplay" usually ranges from mindless grinding for experience to moderately interesting tactical decisions during battle.
Writing is a lot more subjective though. I personally found the slapstick and meta humor extremely funny but felt that some of the inside jokes were unnecessary or out of place. In particular, references to otaku culture appeared pretty often which I felt detracted from the game.