For 2 channels at a time (even at 24bit) the Mbox is solid. The terribly limited USB 1.1 spec is still more than enough to handle the throughput.
Digi has released the Mbox2 recently, which (BELIEVE IT OR NOT) is also USB 1.1!! Not sure what they were thinking in not utilizing USB 2.0, but hey it still works.
Video is a lot more intensive than audio, so dropped frames would be common on a USB1 interface. Audio 2-4 channels (simultaneously) should be OK though.
Dirk