I did this for a few days a while back when Fab.com was offering the credit for new referrals on both ends (money for them, money for me) but wasn't able to get it to ROI positive. There were a ton of sign ups but not enough people made that first purchase to get me the credit and they're pretty smart about expiring credits and the putting in a 30-day limit on when the referral must have completed their first purchase.