Hi tpsboston:
Actually what I bought was a mini pci express card (I saw that I didn't put "mini" on the previous post) as that's the slot that was available on the tiny DQ77KB motherboard. Here's a link to what I bought: http://www.newegg.com/Product/Product.aspx?Item=N82E16815158292
When the card is passed through to a VM it works exactly like it would if he VM were running natively on the machine. I ran my tests on it with a Windows 7 32 VM and it correctly recognized the card and installed the right drivers. I was able to plug in and remove USB devices from the card just the same way I would in a non-virtualized environment and the VM handled it perfectly. Performance wise I'd say that the throughput is pretty close to native if not truely native. When I tried copying files to and from a USB 3 hard drive I was getting throughput in the 80 megabytes / sec range, which is in line with what that drive does in a non-virtualized environment. As far as I can tell in every respect that card works with the VM exactly as it would if the VM were the native host O/S.
I've also got an Intel half size mini pci express wireless card installed which is being used by a Vyatta Router linux VM on this same box. It too operates exactly as it would in a native environment as far as I can tell. Linux recoginzed the card and set it up as wlan0 exactly as it would have in a native environment.
Now, the one thing that doesn't appear to work quite right is if I pass through the on board USB controllers to VMs. This mobo supposedly has three USB controllers, at least that's what ESXi shows when I bring up the PCI configuration screen. I ran some tests trying to figure out which USB controller operated which ports by assigning them one at a time to be a PCI pass through device. However, when I did that it appeared as if all the USB ports on the motherboard were assigned to the pass through controller, no matter which USB controller I picked for pass thorugh. This was very strange. Also, the Windows VM never recognized any of the USB ports as USB 3 and always installed USB 2 drivers. Additionally, by assigning any on board USB controller as a pass through device I lost the ability to communicate with the ESXi console via the USB keyboard, even when using the Intel AMT remote control functionality. Fortunately the changes could be undone via the vsphere client. This is why I ended up buying a seperate USB 3 card.
From my testing it looks like installed cards pass through perfectly, but passing through PCI devices integrated into the motherboard is more of a hit or miss process. I didn't test passing through one of the on board NIC cards, and of course couldn't test passing through the disk controller that connects to the datastore, so I don't know if this screwy behavior with on board devices only occurrs with certain devices on certain motherboards, or is more widespread.