I am using the Sonnet card in my 2008 MP and have been very pleased with it. The USB cards out there vary in their features - some offer an internal port or two in addition to the external ports, some have 2, some have 4, some even have eSATA as well. I only needed USB 3 ports, and the 4 on the Sonnet was plenty.
I have had absolutely zero problems with the Sonnet card. I have used it for syncing my iPhone, connecting a USB 3.0 external drive I use for backup, and connecting several high-capacity USB 2.0 and 3.0 thumb drives. The transfer speeds far exceed USB 2.0 and I have been pleased with the speed bump, although I haven't done any benchmarking. I have the card installed in slot 4 (top slot).
I cannot speak for everyone with the same hardware combination, but for my own part I have not experienced any sleep or compatibility issues. The card performed flawlessly and transparently right out of the box, and I would expect nothing less when paying $70 (considering there are cheaper alternatives out there).
I also would not presume to judge what setup might work best given how you have your equipment arranged, but in my own case I found it helpful to buy 2 short USB 3.0 extension cables and run them over the top of the MP case. I attached a small cable clip to the underside of the top handle at the back of the tower and routed the cables through the clip so they do not get lost. With this setup, I do not have to poke around in the back of the computer with a flashlight to find the USB 3.0 ports every time I want to plug something in. I also have a Lightning cable routed through the clip for mobile devices.
