Theoretically, USB Sound card has both merits and short comings.
Merits are the signal is not interfered inside the computer and processed outside, there are chances you get 'better' signal.
However, the USB is managed by CPU/OS, so during some high peak of work load of CPU/OS, your sound card may not receive the signal smoothly. That means you can get a song played in broken parts. But this happens not often.
Theoretically, USB Sound card has both merits and short comings.
Merits are the signal is not interfered inside the computer and processed outside, there are chances you get 'better' signal.
How ...
Many people have had great experience in this product and have never encountered problem as you mentioned here.
Make sure that USB audio is selected for all your playback devices in System Sound preferences. I'm using a PCM2702 based external usb DAC on Ubuntu 8.04 and it works great.