Each faction has a storehouse where you can purchase unique items and spells specific to that faction. Many factions also have a network of contacts throughout the game who can provide you with additional information or help. Each faction also provides you with a special ability:
ANARCHISTS: you can infiltrate other factions and pose as one of their members, gaining access to many faction-specific quests and items (but not other special abilities from those factions)
DUSTMEN: you gain a bonus to armor class vs. undead creatures
GODSMEN: you gain a Charisma bonus
SENSATES: you gain the Sensory Touch ability (heal another while damaging yourself), as well as access to Sensate-only sensory stones which can restore more of your memories and provide extra experience bonuses
XAOSITECTS: no special abilities per se, although you'll be able to freely walk through some hostile areas without being attacked
Good info. Can you recomend a good place to look up the storehouse/contacts stuff? From my in-game experience the Sensates is my style. Gonna join them soonish.