While I love Mage, one of the drawbacks was the sense of a lack of horror to make it part of the WOD system and settings. There wasn't or isn't enough of the dark, edgy horror of the unknown.
Intruders fixes that quiet nicely. It offers up a wide variety of threats and encounters all related to the Abyss and bringing a way of horror properly to the game.
Some of the encounters seem pretty straight forward and easy to deal with. Others, like the House, are far more complex and harder to deal with.
For the Storyteller, there's nothing but page after page of good ideas for a Mage Chronicle.