You only want to use it if you want a more robust payment then a 'buy now' button. In other words it's a lot to deal with if you only need that level of functionality. My app has a subscription model, and will be expanding out to a market at some point in the future. FPS handles all of those models. They also provide some good guarantees. Given some of the recent complaints about about google payments, and my own personal experiences with paypal I'm beginning to value these guarantees very highly.

However, my system is not fully implemented and deployed (hence no link yet), so I'm not ready to provide a full endorsement. I can only say that the experience so far on the development side has been very good, and the sandbox makes extensive testing before deployment very easy.

I will report back when I've deployed the system in production.