In my honest opinion, Twilight Princess is NOT worth buying. The game's plot seems wholly unimportant -- nothing really happens. The gameplay is great, but the enemy AI and difficulty both are so EASY that plowing through an entire Hyrule Field worth of Darknuts hardly seems a challenge. You barely take damage from attacks that seem as if they'd decapitate Link; I know this is a gameplay mechanic and the game doesn't have to be REALISTIC, but when you have all 20 hearts and get hit only to have 1/2-1 heart chipped off from the likes of a DARKNUT, the game feels really underwhelming. The game is too big with not enough content to do. The best aspect of it IMO is the scenery, though what you'll be looking at the majority of the game is dull browns, greens and greys. Oh, and let's not forget that TP is ~99% OoT rehash with less substance.
The only time I think TP is worth buying is if you haven't played Zelda: Ocarina of Time [3D].