Despite so many aliens, games... eventually start feeling the same. Which is ironic, considering the main point of CE is that it should never feel the same. Downtime starts getting too high once you add a sixth player, and is nigh-unbearable with seven.
Good for about 4-5 hours of fun, if you have that amount of time to spare in the first place. More fun for the heroes than the Overlord, but hopefully you have an experienced group of players who can rotate Overlord duty with you. Quests are frequently imbalanced, some seem to be impossible unless you've played before and know where everything is, others are way too forgiving.
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.
So easy to teach (everyone plays Poker, right?), intuitive, light and doesn't take very long unless playing with people who manage to AP on die rolls. Not much strategy here, but for an hour of dice, better than most.
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.