I bought a used D&M unit from a guy on the "other site" for $350, before I even had my bike.
There was a lot of controversy regarding how well they work, particularly as the speed increases, and whether they had to be mounted vertically to avoid any "ram air effect." Mine has worked fine mounted horizontally, showing no such symptoms.
You will have to rig something up with the enrichener, which is an integral part of the BDM air cleaner. In the picture below you can see a knob just below the left arrow. Frankly, this is not a big deal, since I never use the enrichener anyway - but you do have to do something with it.
Regarding how open it is - there is a small space between the filter itself and the interior surface of the spike cover, so essentially the entire filter surface is exposed. That is, the air flow is not restricted at all by the chrome spike cover - it just supports the filter element on the front and back. If you look carefully in the picture below, you can see the gap just below the right arrow. I suspect the one on eBay is similar, though it is hard to tell from the pictures.
It sounds like several people want to try one - it may be a good item for a group buy, if someone wants to take the lead and do a little negotiating.