Really a mixture of both. The single-player is actually decently lengthy, especially if you attempt to 100% it. But I mostly am referring to the multiplayer. It's addictive like no other multiplayer game. Usually I stop playing a multiplayer game after a month or so (like Overwatch, for example), but Splatoon just has me hooked. Being able to customize your inkling to your liking with gear that have unique abilities, along with the huge variety of weapons, subweapons, specials. The new stages, weapons, and even modes (in some rarer cases, see: Clam Blitz) constantly being added. It's fantastic. And since matches are only 5 minutes in ranked mode and 3 minutes in regular mode, they're over in a flash and you just can't help but be like "just one more match before I go to bed!"
And with the Octo expansion coming out later this year, Splatoon 2 will probably remain one of my favorite games of this year as well. If you've never played a Splatoon game, I highly suggest trying it out. I think most of us Nintendo fans here have played a Mario Kart, Mario platformer, or Zelda game before. And so the Switch entries aren't THAT different (Odyssey is Super Mario 128, Mario Kart has a new gravity mechanic and 200cc, but it's still Mario Kart, Zelda is now open world, although it's kinda always has been, it's just now less linear, kinda like a larger version of A Link Between Worlds) but Splatoon is a bit newer to the Nintendo world, but has just as much to offer.
EDIT: If you have a Wii U and haven't yet used Gamefly before, if you sign up they let you rent for free for the first month. Rent the original Splatoon and play through the single-player if you want to catch up on the story, before playing Splatoon 2. It's not necessary, but it does make you appreciate some of the characters more, like Callie and Marie. And if you play some multiplayer matches I think you'll have a decent idea of what Splatoon 2 is like. It's what I did before Splatoon 2 came out.
Oh, and use motion controls. They're great.