Burning Crusade is an excellent expansion to an excellent game. Definitely worth your time and money.
Burning Crusade introduces two new races into the game, the Draenei for Alliance, and Blood Elves for the Horde. The new races have their own homelands which offer up some fresh questing for players who are tired of the same old quests in the regular lands. Blizzard decided in the expansion to give Alliance access to the Shaman class with the Draenei race, and alternatively, gave Horde access to the Paladin class with Blood Elves. There used to be a lot of complaining on both sides (Horde and Alliance) about each respective class being unfair/unbalanced, but that debate is pretty much over now that Alliance or Horde neither one has any perceived advantage in class choices.
The other huge addition to the game in the expansion is the addition of the Outlands, a new land area for you to explore. I believe you need to be level 58 to enter the portal to Outlands. Also, the level cap has been increased from level 60 to level 70.
There has been a ton of content added to the game through patches and the expansion that seems to keep offering new things for players to enjoy. While not without its flaws, WoW is arguably one of the best MMO games to date. An unfortunate side-effect of introducting new content such as new raid dungeons and raid items, is that a lot of the old content is now ignored because its simply not seen as worth the time or effort since the new stuff in Burning Crusade is leaps and bounds better. So that is unfortunate. Some good dungeons/instances rarely get done anymore because of it.
But overall Burning Crusade is an excellent expansion and if you own World of Warcraft, it is definitely worth the money.