In computing, DirectInput is a Microsoft API for collecting input from a computer user, via input devices such as the mouse, keyboard, joystick or other game controllers. It also provides a system for action mapping, which allows the user to assign specific actions within a game to the buttons and axes of the input devices. Additionally it handles force-feedback (input/output) devices. Microsoft introduced a new input library called XInput in DirectX 9 (December 2002) specifically for the Xbox 360 controller.
-wikipedia
XInput is an API that allows applications to receive input from the Xbox 360 Controller for Windows. This document describes the differences between XInput and DirectInput implementations of the Xbox 360 Controller and how you can support XInput devices and legacy DirectInput devices at the same time.
-MSDN
Now jim.. you need to understand one thing... I'm the evil Bill, So don't try to put me down because I like to share, I'm just not as refined at my age. But the fact is I own all three of those controllers as well as a 360, and I know that if you buy a game off steam you best look for the icon that indicates it supports the 360 controller. Not a problem since most games were initially written using the Xinput libraries these days because they were intended for the 360 not the PC. So no you just can't assume your 360 controller will work with every game you buy for the pc. And no FSX doesn't take advantage of the full capabilities of the 360 controller it's an emulation at best. :)