here are multiple things that makes this game a must buy.
First one - the comedy story. The game is so humorous that you will gladly face chapter after chapter even if you have no friends online to share the experience with you. The plot is crazy and leaves you questioning "what's next?" a lot. Couple that with some crazy achievements (such as "This is Sparta!") and suddenly the game is worth playing even if you're alone.
Second one - the unlimited combinations of spells. You're given runes. Runes to determine how strong the spell will be, what it does, how it casts... They won't directly tell you what they do, because you see the runes are simply elements. It's your combinations that will determine the effect.
And the finally - the fun of destroying your friends while playing co-op. Feeling fiendish? Put some fire into that heal spell. I'm sure that your friend won't mind... Oh your friend found that awesome staff! Want to get it for yourself? You know what to do!
Consensus: can be played alone for the humour of the story, but best played with friends because of all of the destruction to each other that you can cause.