I don't have it yet, but I have seen a few reviews and gameplay videos on Youtube of it.

Apparently there are three modes: Story, Multiplayer, and Practice. I'll go ahead and tell you that Multiplayer is going through a level once, and then handing the controller to player two for him/her to go through the level too. Practice mode is where you can do any of the levels you've beaten in story mode. Basically the mechanics involve using the Wii Zapper to stand and shoot in a fixed location in one direction, then do the same but you can rotate 360 degrees, and the last one is where you can walk around using the analog on the nunchuck, while aiming and shooting with the Wii Zapper (this is what most people complained about because it felt so awkward). You are trying to shoot Legend of Zelda themed enemies in LoZ themed worlds is the whole objective.

As to if the Zapper is any good, again, I can only tell you from what I've seen and heard on reviews/gameplay videos. Basically it is good for on-rail shooters or modes in games, but when it comes to moving around while having to aim it feels too awkward and you might not like it.