Nyarlathotep is a very mixed bag as an AO. On the one hand, he can be disgustingly easy to defeat in combat, even if you aren't especially prepared for him. Because they go into the monster cup you could go an entire game without seeing a Mask, and his Endless Cultists increase the difficulty of drawing a Mask monster. On the other hand, the eight Masks are very fun and challenging when they show up, and Nyarlathotep has the coolest Sinister Plots of any AO.
Whenever I do a game against Nyarly I always sprinkle the Masks across the top of our monster cup. Usually this pays off with increased exposure to the Masks, and the current game is no exception. We had the Bloated Woman appear when the first gate opened at Whately Farm. She passed through a vortice, and showed up later in the game at Unvisited Isle. A closed gate sent her back and she showed up AGAIN at the Black Cave, just as Diana was trying to close and seal the gate! The Haunter in the Dark was actually driven off by the bright and sunny evironment (the first time I've ever seen that happen), but he too returned. The Black Man has shown up and been defeated once, but here' s the really impressive part. The Beast appeared from an encounter at the Graveyard and nearly devoured Diana and the Skinless One has shown up on two different "A monster appears", once at Hibb's Roadhouse and once at the Science Building! In neither case was the Investigator involved remotely prepared to confront him and many clue tokens had to be sacrificed in order for us to escape.
Right now we have four gates sealed, and things are looking good, but we'll see how it goes. Anyone have any stories about encounters with the Masks?