XPS 8950, add in PCIe USB expansion card?

Has anyone confirmed what make/type/model of USB expansion card will work with the XPS 8950 PCIe x4 slots? I have a FebSmart card which does not work. I have also read on this forum about several other issues with different cards. I also saw one post where Dell was "working on the problem". Is there a resolution and is there a recommended card(s)?