Personally, I think that FE and FES is a nice piece of software.
Downloaded both from Apple Store this monday, I have produced and release for test an application that needs, using xCode, weeks of more depth development.
With FE, I am ready to distribute this to some users for testing the main core functionalities directly on their devices, thanks to FES, without any kind of local synchronization with my machines.
This is an important and fundamental part of developing an iPhone/iPad application, testing on devices.
I can do this also whit xCode, surely, but if you want to release a beta, i think this is the way.
Obviously, the final product must be developed in xCode, adding Apple's API that FE doesn't cover (localization etc..) and with more UI personalization, but for the main objective (filling a very long information form for manage first aid healthcare) it was fundamental for me, and for my testers on field to understand in which way the application can move on, also in a right "client/server" structure, that FES give by default (identification and register on field devices, sharing documents etc.)
In conclusion, FE and FES can work closely with xCode, especially in the first step of development, as a toolbox to release functional mockups of the application you want to develop.
And for minor complex applications, obviously, that can cover, as suggested from Widgetpress website, on field informations workflow, is a must.