You can make some good money with clickbank, granted it is a different beast with CPA offers. Right now I am about even with CPA offers and the clickbank products I am pushing.
Because you are sending customers to a payment processor like clickbank that they are not familiar with and they have to whip out that worn out credit card, you will need to do your homework as far as pushing your offers to the right audience.
It's dead simple to get people to fill out a free form, but to get them to spend their last dollars will take some convincing and knowing how to get into their heads.