It's a good game. It's just not a good GBA game.
It's a big change from the normal style of Zelda games because it is mainly 2D. This is not necessarily a bad thing. It does raise the difficulty of the game, because you can't just go around enemies like in the 3D Zelda games and you have a higher risk of falling off ledges.
This game is not without its share of secrets. There are secret caves and areas which contain extra heart containers and magic containers which increase your max health and magic respectively. There are places you can visit to learn new magic spells that help you jump higher, reduce your damage, reflect attacks among other things. There are places that you visit to learn new sword techniques like the stab down technique. You visit new places as you progress through the game and you collect items that make your gameplay easier. For example, somewhere in the game is a Magic Key which will eliminate all your problems with locked doors.
This is a great game, but it has a big drawback for being on the GBA. For one thing, very few changes have been made to fit it on a handheld console. The graphics and sound are great for an NES game, but in order for it to be a good GBA game, there has to be a more significant change. Also, if you have it for other systems such as on The Legend of Zelda: Collector's Edition for Gamecube, you will not want to pay a lot of money for another version of the game, even if it is for a handheld. Maybe if it were $5, it would be worth buying.
On a good note, the replay value is very high, because if you beat the game, you can replay it keeping all your spells and sword techniques making the game much easier and even more fun. It's a game you will enjoy, but it's just not good enough to buy at full price.