We're looking into PayPal, but I try to avoid systems that put strict limits on what I can do with my customers. Things like refunds, deferred charges, adjusting price up and down, etc. I can do all that easily with Chargify, but I'm not so sure about PayPal.

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