I recently replaced my first 5050 with one from ebay. My older 512 cards (xD & CF) were fairly similar in write times. The ebay camera has a newer 512 CF and the write time is signifigantly quicker. I am not convinced the difference is all in the card, some 5050's may perform better than others, but the old xD is not as fast as the new CF in the new camera.
I shoot a lot of wide angle ambient where strobe recycle time is not a factor and the faster write time is a welcome improvement. When using my strobe I use MyModes with manual settings, including fixed focus, so I do not wait for AF, I wait for the card. My strobe is an Ike SS 200 at 1/8 or 1/4 power so I have no recycle wait, even on high continuous. After a 4 image burst, I wait, wait, wait, wait for the card to write. If the difference is 1-2 sec per image I am shooting 4-8 sec sooner with the faster card.
A 512 CF is 68 raw images (69 xD) so unless I do a 3-tank boat trip (Niihau) capacity is moot, and I still have the xD for backup. Herman may see speed as no big deal and capacity as the only real advantage but I see faster write speed as the biggest advantage. Even users with identical rigs and similar styles may see this issue differently.