The multiplayer in this game is some of the best out there. Customizable game modes, a map creator, and lots of items.
However, this game isn't perfect. For one thing, the platforming is terrible. If you fall and take damage, costing you your turn, half the time the controls are to blame.
Then there's the single player campaign. The AI is the very definition of perfection, and is some of the most unfair AI I've ever seen, and I've played a lot of Mario Kart in my time. You could be on a tiny floating chunk of land just big enough to stand on and an AI worm can throw a grenade from across the map, ricochet it off walls and celings and get it to land perfectly on that worm's head without bouncing off. Not only are they unfair, but as you progress, they start to outnumber you more and more. I've gotten to the point where I've started with only 4 worms and the AI has 10. Unless you're the kind of person who enjoys games like Dark Souls, where the challenge is based on how unfair the AI is, I wouldn't touch the singleplayer with a 10 foot pole.
That aside though, this game is clearly meant to be played with friends anyways, and it's definitly worth the buy just for the multiplayer.