These cards don't work well, giving <100Mb performance, in Linux and freeze up the machine in ESXi. So, don't try them in those circumstances. I got them from the previous owner for this specific reason. He had three Linux servers and these performed lower than his expectations. So he sold them on ebay.
They work very well in Windows 2008 R2 or Windows 7. Under either of these OSes, you can bridge the ports and make the system.server into a poor man's 10Gbe switch. Great for home use, if you need fast access to your files from one system to another.
I used them like this: Server was set up as a router from the cable modem, with both 10Gbe ports bridged to make it into a switch. HTPC was close to the server so it used the short cable directly connected to the server. Main machine was in the next room, so I used the 10m cable to connect it to the server. This setup dropped 5-10ms from my latency to WoW compared to the onboard RealTek and Atheros cards, and allowed me to play back BluRay MKVs from the server through the HTPC to the TV without any stutter. It was nice and all, but I wound up needing my slots for other purposes, plus I changed my server over to an ESXi 5 host, which didn't like these cards no matter what drivers I used.
for the whole set, I'd like to get $350, (I paid $500) but email me with an offer and I'll consider it.