Honestly, I haven't looked closely at the raw A.net API. I heard about Chargify, pricing looked good, and it fit my model. Basically, I wanted someone else to deal with dunning, expired cards, running the periodic charges, etc, etc.

I write about it more here: http://peachshake.com/2010/06/15/saas-subscription-billing-o...

Up to now, I've been nothing but pleased with Chargify. This change makes me question their sanity a bit, though.

Here is the PDF with the authorize.net recurring billing integration information:

http://developer.authorize.net/api/arb/