Fallen is Babylon is the first Storyteller Adventure System (SAS) module I've ever run and I'm extremely impressed by it.
The SAS contains eight tightly designed scenes based around the infernal pacts of Dr. Rasoletti and others. As with other SAS products, Fallen is Babylon includes extensive background material, NPC character sheets, discussion of the scenes, and scene summaries and player handouts at the end.
What makes Fallen is Babylon such an impressive adventure is the efficient design - author Matt McFarland was able to create a complex and evocative plot with only two mortal and two infernal characters. The first half of the adventure requires almost no work for the storyteller - in my experience the SAS gave me every recommendation and description I needed. Towards the end, as the players come up with their own ideas and schemes, the SAS becomes a bit more open ended. Don't expect prewritten plot points to get in the way of your players.
Overall, Fallen is Babylon is great and worked perfectly as a four hour one-shot story.
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If you'd like to see an actual play of Fallen is Babylon, check it here: http://www.youtube.com/watch?v=I4aX5PbZ1F0