Short version: * Sane well thought out APIs that let you do stuff very, very quickly and easily * You don't have have a merchant account - so the time taken t

Short version: * Sane well thought out APIs that let you do stuff very, very quickly and easily * You don't have have a merchant account - so the time taken to get payments up and running drops from weeks/months to almost nothing * Attractive fee structure - especially for bootstrapped projects (you don't pay them until you get paid)