Not only should you buy the Wind Waker, I'd argue it's easily the best value you could get out of a Zelda game.
The main quest isn't tremendously long--it spans 6 major dungeons at most, and one is the requisite "final dungeon," which means it isn't all that extensive. That said, it's filled with vibrant characters and some of the most clever items/gameplay styles around. It actually fixes some of the problems I've had with the Zelda franchise in regards to pacing, and introduces some new items.
But it's the sidequests where the game really shines. There are numerous islands to explore. The Great Sea itself, contrary to popular belief, is not empty at all; each square is filled with treasure, mystery, and its own unique story. A lot of the items/upgrades you'll want will be difficult to find, because the game does not hold your hand on sidequests. Furthermore, Rupees actually count for a lot here, so when you've got a wallet with four thousand rupees in it, you WILL find a way to use them. This makes exploration even better.
If you only buy one Zelda game, I would always recommend Ocarina of Time first, but The Wind Waker is unquestionably a great game for your money. It's also my second-favorite Zelda game. Buy it.