I guess I'm somewhat obligated to answer this query, my name being on the credits and all.
The game is hella fun with multiple people. Singly, you can do the whole story thing or run around the multiplayer mode by yourself, whatever floats your boat.
The only thing is the solo game is easy. Like, no-fucking-brainer easy. Once you figure out the OoT style puzzle-dungeons the game devolves into hack-n-slash beat-em-up with a character that's actually difficult not to overpower with.
Multiplayer is a little better, since the monsters are of course powered up, and even though they're still just various sizes of fodder it's still a blast to mow them over with a small army. You get the exact same dungeons in multiplayer that you get in solo, but they're altered slightly to suit and challenge your party makeup, which leads to interesting situations. Oftimes you'll run into a Lost Vikings type of situation, where everybody needs to use their powers just the right way to make sure that the whole party can proceed. It's usually simplistic but still satisfying.
There are also missions in Multiplayer, which tend to be pretty bullshit (FUCKING CATS GOD DAMN IT) but are nice if you're into coordinated efforts and such.
Oh, and if you do run Solo for the plot, be warned that it's pretty fucked up. I actually made a post about this game back in the Childhood Trauma thread when I was testing it (HUGE SPOILERS DON'T GO LOOKING FOR IT).
So yeah in short this is a game for people with friends, definitely. Friends with DSes and their own copies of the game, which might be a little limiting. I'm not saying it's a BAD game for lonely people looking for a simplistic Action RPG, but you'd probably be served much better by Crisis Core or The World Is Awesome The World Ends With You.