I was feeling really bored just now and did a little experiment. Actually this was spinned off by my sister's request over the weekend to help her take a mugshot for her passport photo. I was trying to use a Stofen Omnibounce over my flash and found that the effect was really bad for portrait shots, since the flash is coming in from the left of the lens, it keep casting an ugly shadow on the right of the wall.
I think this picture says it all.
Left: Direct flash 90 deg upward onto ceiling
Middle: Direct flash 90 deg upward with bounce card face forward
Right: Stofen Omnibounce 90 deg upward
I think it is clear which one has the best effect for portraitures. While in this pic the left picture delivers the best result, for human face, that would have resulted in harsh shadows under the eyes, nose and chin. With a bounce card, the effects would be better with fill flash into these shadow region with minimal rear shadow on the wall.
Looks like I need to make my bounce card soon, since I was just holding an A4 paper behind the flash for this shot...