The Old Station is still used as a car park, although the westernmost section (the original Brunel bit) is now an exhibition space.
Perhaps you have not heard, but it seems very likely that the Old Station will return to rail use in conjunction with the GWML electrification and demolition of the current Bristol Panel. I rather suspect that this won't include the Brunel section - the catenary wouldn't enhance the view of the pseudo-hammerbeam roof.