I wasn't too impressed with USB audio in Windows XP and Vista. I had to manually switch between audio outputs, and I saw a few blue screens when unplugging USB audio device.
USB audio finally works right in Windows 7. When a USB audio device is plugged in, audio output switches to that device. When the device is unplugged, audio output switches back. I can even use my USB wireless headphones to listen to line in on the motherboard. Most importantly of all, USB audio doesn't crash Windows 7.