Just wanted to share (I know this topic is all time popular) about our experinece with Amazon FPS.
First of all, we charge a small subscription fee each month. This severely limited our abilities to cherry pick payment processors. Second, we did not want to deal with (store on our servers) sensitive data such as credit card numbers.
From a technical point of view FPS is a bit too complicated. Definitely more so than other gateways we looked at. That is because it's too generic: instead of 2 perties there are always 3. "Build your own PayPal!" is their idea. For people who aren't building their own paypal it is a bit annoying.
Secondly, your users must have Amazon accounts. That may be good and bad, depending on how you look at it. To us it was bad: we did not want people to see "Amazon" stuff during sign up process - we had some unpleasant experience with similar approach taken by PayPal.
clipped from: news.ycombinator.com 