I have a Driving Force Pro, (well, it's about 500 miles away currently, but I can more or less remember my button map)
I have the D-Pad set to look around the car (left, right and down) and fire the ignition (up), Start and Select are horn and lights R1 and L1 are the driving aids (TC, Speed limiter), Shifter is the shifter, paddle shifter is the clutch and handbrake, Circle Square Triangle and X are mapped to chat functions.
These are the ones I can remember right off... Other people might prefer having other things mapped, and remember if you use a stick shifter put the button clutch on the opposite side to the stick...!!