I use CardOffers on my credit card site...
There selection and payouts are great. There methods of getting product to you is great as well, HTML Templates, JS, Spreadsheets and they just rolled out an XML Feed.
The downsides is that there customer service is pretty much non-existent, the only time ive ever heard from them or get a response from them is when they have something to complain about with the ToS or rates being out of date.