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.
But most importantly there is ZERO customer service. They have no phone support. None. If you try calling Amazon you won't find a single person who knows anything about FPS. And their "Technical Support" form WILL NOT return back to you within 24 hours. Basically it means that the system is not quite ready for production use: if your customers cards get rejected for no reason, you will need to solve that problem NOW.
Well.. that's about it. Just wanted to share, since I picked up FPS advice somewhere on this board. We're going with authorize.net although it's unclear how easy it is to work with them using Ruby.