The Orico PFU3-2P does not require supplemental power to operate. I have tested this with flash drives and 3.5" external drives that have their own power supplies.
It might be possible that there is insufficient power for a high-current device such as trying to charge an iPad off of it. In that case, I'd ensure there is a powered USB 3.0 hub in between the card and the device, or use the MP's built-in ports.
The main advantages of the Orico are that it is very inexpensive (about $18-$25) and it uses native drivers built into OS X, so there is no dependance on downloading third party drivers from websites and less worry about future OS support.
The main disadvantage is that both ports on the card are connected to the same controller, so hooking up two drives and using them simultaneously really slows things down.
I personally use the PFU3-2P and it works well for my purposes. I use it for USB 3.0 flash drives and USB 3.0 External Desktop drives. It also works with the USB 3.0 hub built into my Dell Ultrasharp.
There are some other things you should be aware of:
(1) While transferring date, USB 3.0 can interfere badly with Bluetooth. This is a known problem in the industry and is not limited to just Mac Pros with add-in cards. The Mac Pro is affected more than most because its Bluetooth reception is notoriously weak to begin with. If you use Bluetooth devices, be aware you might lose reception, which is fixed by either adding an external antenna or adding a USB Bluetooth dongle.
(2) Some of the more expensive cards are for "storage devices" only and won't connect to other types of USB 3.0 devices. People discovered this when they tried to add a hub, couldn't, and were told by support that the cards were for storage devices only. Sorry, cannot remember which one has this problem.
(3) The Orico card will work with many hubs, but not all hubs.
There is a thread that toward the end has some compatible hubs listed.
