It's a gigantic dating sim if you're doing the pacifist run, and it's the easiest RPG ever if you do Genocide (there are only two big obstacles in the way, and the rest is fuck easy). The bullet hell dodging is a decent choice for a RPG, but at the same time kind of easy. Like, I only died at the neutral run's end during my first playthrough, only because I didn't want to fight Asgore.
I do like the game, either way. It's pretty creative and the way it manipulates your choices and the consequences of it (through saves and resetting) is really unusual to see. The character designs are also charming, and the idea that a human child resurrected from the dead can be a complete sick fuck entertains me.
But the gameplay?... Well, as Inuyasha said, the game is more story based, rather than gameplay based. You might as well watch the game on Youtube, because unless you like getting surprised by yourself with the choices you make, you are not exactly missing out on the little gameplay it offers. I'm not saying it's a bad thing, but the game is based on making choices with this and that and not much beyond that.
I do really wish that the Genocide run was more realistic with how serious of a threat the monsters were dealing with. If there's a unstoppable murderer on a rampage to kill everything they can find, you'd think that someone in a position of power would've prepared an emergency plan to stop such a blood thirsty being, have an army prepared and send it right into this serial killer with strong tactics in mind... but nope, everyone is just chilling until you find them and kill them off, because nobody gives a shit. The most they can do is just hide, keep a few guards in the way and wait for the inevitable.
The typical concept of a "genocide" is not really meant to be easy, but this game hands it over to you on a silver plate, given that nearly everything dies in one hit (you can mostly do it with a fucking stick, if you want). It's the lamest genocide ever, given the only decent thing in it is exploring the different side of few characters and that's it.