Quests are frequently imbalanced requiring house rule fixes - although that could be said for a lot of other stuff in Descent. Treachery is cool, but some of the other additions (e.g. power potions, some of the new treasures) seem a bit unnecessary. Still a solid addition, should be added into the base game.
Makes the game from above average to great - no longer just a gateway game but rather a light strategy game that's easy to pick up but even experienced gamers can enjoy strategically. I'd consider this essential.