I have tried one of those in my P55 system with external power supply, testing all jumper settings and found the onboard ports sounded better. Results might be better in a fully optimised dedicated music server but I have yet to build such a machine.
But at $300 you are much better buying a dedicated music server such as this
one of these which actually contains the SoTM card as well as a CPU, motherboard, ram SSD and optimised Linux install for only $300 more, although this still doesn't mean that the SoTM card is any good.
All of these sources however are suspect as they are hosted on sites which distribute SoTM products so I would be cautious -
none of the CMP² forum dudes use USB controller cards, but their rationale is pretty different.
The issue on my system may well be to do with the PCI bus sharing etc but unfortunately all the motherboards I wanted to try a music server build on are either out of production or yet to be released...