Some of my friends have subscribed to ActiveAdvantage.  From what I understand, the leading benefit of being an Advantage member is saving on active.com processing fees when you pay for a race entry fee online.  If you register for a large number of races each year, those processing fees can quickly add up.  You also receive discounts with various sporting goods vendors as well as access to free on-line training programs, but I think the main selling point is the discounted processing fees.