I have several affiliate payday loan sites with high rankings, as well as several financial blogs which are used to advance my main money-making sites. Obviously, I don't want to give away all my secrets, but I incorporate many of the strategies discussed on this forum...
For the record... I've attempted marketing credit cards in the past as well. I'm sure it's very profitable for well-established credit card websites that have been around for years. But it's extremely hard to break into the Top 10 for any high-traffic (or even secondary) keywords with so much established competition. Plus, the competition is made up of VERY LARGE marketing companies with EXTREMELY BIG BUDGETS. It's difficult to be competitive in SEO or PPC with so much money being thrown around... Not saying it's impossible, but it's extremely difficult. Payday loans are also competitive, but it doesn't seem to be dominated by huge marketing companies... JUST MY 2 CENTS