Lionhead has learned from past experiences and has managed to pack a lot of action and comedy into Traitor's Keep. For 560 Microsoft Points you'll get a great but short adventure that follows the Fable III storyline.
You are looking at about 3 to 4 hours of playtime with this add-on pack, not terribly overpriced for 560 marketplace points ($7). The main draw is obviously the expanded storyline, ideal for Fable III junkies that want to stay engrossed in the narrative.
I can recommend this add-on to everyone, especially the ones who have finished the game and would like to spend more time in Albion. It is a pity though, that the decision making mechanic isn't extended – the developers kept what was in the original game.