The default force feedback in FS is lacking in many ways, and in my opinion does some things rather horribly and annoyingly wrong. For example, the turning force that it does. I feel like I am fighting my joystick constantly whenever I make a snap turn. It just feels wrong.
What FSForce does is completely replace all default force feedback controls. One example is when you are about to stall, it automatically calculates for the aircraft and you can feel your shake rattle before the onset of a stall. There are forces present for taxiing on concrete, so you can feel little bumps, etc. There is even gear buffering. The best thing about it however is you can set up profiles for all planes you want, so they perform differently, such as high speed vibrations during takeoff, etc. Centering forces, etc, all can be customized for your personal preference.
FSForce takes up very little background cycles. Everyone I know who has it has not noticed any difference with it running in the background, myself included.
Of note, I have done no commercial work for the maker of FS-Force, so I am speaking simply as a very happy customer.