There are two modes -- Battle and Adventure. Let me go through each:
BATTLE -- The battle mode requires more than one player and is a blast. Basically there are up to 10 one-screen-maps that you can play strategically to try to clobber your enemies by making them lose their hearts. Each map is done VERY well so that the game is a real strategy. If you do this, I can do this, if you do that, I can do that to combat it -- and if you combat it, I can do this to combat your combat. (If that doesn't make sense, trust me, it's a heck of a lot of fun)
ADVENTURE - This can be played by 1-4 players and is a blast as well. Figuring out puzzles, beating bosses, collecting force gems, and clobbering enemies has NEVER been funner on ANY Zelda game.
The reason why? It's multiplayer! The fact that your cooperating and competing with your freinds make this game the funnest gamecube game on my shelf.
Another nice thing with adventure mode is that the game is divided into many smaller puzzles so your not stuck playing for 4-5 hours whether you like it or not. You play for about 20 minutes until you finish the section and you can take a well deserved break.
There is one thing that I thought would be a disadvantage -- the GBA connection requirement. This turns out to be an advantage and a disadvantage at the same time. The disadvantage is the fact that not as many freinds can play, but the advantage is that it enhances the gameplay immensly.
As for the single player mode, I haven't tried it -- but if you even only have 1 other freind with a GBA and a cable, this game is a MUST x 100.