Masks is by far one of the most interesting and compelling adventures I have had the pleasure of reading and playing. I don't want to divulge too much of the plot for the sake of those who will still end up playing the game, but I will try to at least mention the high points of this adventure.
Masks presents a strong investigative / pulp adventure vibe, and while it is still the traditional and horrifying jaunt through the Cthulhu Mythos, there's plenty of travel, research and adventuring to satisfy the needs of many a gamer who enjoys the era of play. The presence of so many varied and interesting NPCs each with their own secrets and agendas that can run counter to the players in the most interesting of ways.
Masks is a tightly linked series of adventures that forms an entire campaign, and as such, it requires a lot of dedication from its players. This isn't a beer and pretzels adventure by any stretch of the imagination. Masks requires a lot of attention, care and note-taking to get through, and sometimes even the most careful investigator might end up in over his head.
I would highly recommend Masks of Nyarlathotep as an essential part of an CoC library. The campaign itself is long, and at some parts quite possibly unfair, but for a dedicated team of Investigators, it's all worth it for the memory of being able to take on Nyarlathotep and live to tell about it.
[5 of 5 Stars!]