Here's a quick test I did yesterday:
http://www.talkphotography.co.uk/forums/showthread.php?t=34489
Not day-to day portrait stuff but I'm happy how it works.
Ideally You'd need a couple of these, running wireless. 2 x SB800's maybe or even a simple cheapo slave unit running on one of them.
This was linked to the D2X with the Nikon cable, so it was running semi-auto from the camera's control.
Mostly that was ok, but of course the flash output isn't going to be constant then. Full manual would be better if you want more controlled conditions. Saying that, If I was operating the camera properly, (rather then trying to hold it in one hand to take my pic) there would be more control, plus the ease of the Nikon controlled flash setup.
I did boost the flash a bit though to make up for the diffusing screen.
All in all, a good purchase I think. I'll get another or maybe one of those ebay ones to see the difference in construction.
I think you'll certainly be able to do more with this setup than the brollies.