FPS provides a robust set of interface tools to the amazon payments system. It enables a number of payment models that other services don't offer such as, subscription, aggregate (i.e. micropayment), marketplace/3 party (i.e. user to user transactions with optional commission), and the prices are competitive. You can even do user to user transfers with no fees. They also offer fraud protection.

It is not trivial to implement in an application but does seem to be very concise for the features it provides. The con for this flexibility naturally is increased complexity. They do offer a 'Simple Pay' solution if you don't need the extra features, but I've not used that.

During development you can tie you application to the FPS 'sandbox' which simulates the complete user experience as well as virtual payments and fees so that you can see exactly how things will work in production from various points of view with out actually moving money around.

I needed subscriptions, so I could not use the other popular solutions (when the decision was made), and therefore can't compare. However, I refuse to use paypal because they put 100% of the risk on the account holder, while at the same time not disclosing information about the purchaser to allow for fraud investigation, this is flat out unacceptable.